The answer is 2 Reserved Instances plus Auto Scaling with On-Demand Instances. This combination minimizes cost by using Reserved Instances to cover the steady-state baseline of 2 instances overnight at a significant discount, while Auto Scaling dynamically adds On-Demand capacity during business hours to handle the variable workload without any upfront commitment. On the AWS Certified Cloud Practitioner CLF-C02 exam, this scenario tests your understanding of cost optimization strategies—specifically how to pair Reserved Instances for predictable, continuous usage with On-Demand for elastic, short-term spikes. A common trap is selecting all Reserved Instances, which would waste money on unused capacity overnight, or all On-Demand, which misses the discount for the baseline. Remember the memory tip: “Reserve your base, scale the rest”—lock in the minimum you always need, and let Auto Scaling handle the peaks.
CLF-C02 Cloud Technology and Services Practice Question
This CLF-C02 practice question tests your understanding of cloud technology and services. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company runs an application with variable workloads. During business hours, they need 10 EC2 instances; overnight they only need 2. Which combination minimizes cost while meeting this requirement?
Why wrong: Running 10 On-Demand instances at full capacity 24/7 pays for idle capacity overnight — maximum cost with no optimization.
B
10 Reserved Instances
Why wrong: 10 Reserved Instances commits to paying for 10 instances regardless of whether they're needed overnight.
C
2 Reserved Instances + Auto Scaling with On-Demand for variable demand
Reserved Instances cover the predictable 2-instance baseline at lowest cost, while Auto Scaling adds On-Demand capacity during business hours and removes it overnight.
D
10 Spot Instances with Auto Scaling
Why wrong: Spot Instances are appropriate for interruptible workloads but are not reliable for business-critical applications requiring guaranteed availability.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
2 Reserved Instances + Auto Scaling with On-Demand for variable demand
Option C is correct because it uses 2 Reserved Instances to cover the baseline workload (overnight) at a lower cost, and Auto Scaling with On-Demand Instances to dynamically add capacity during business hours. Reserved Instances provide a significant discount over On-Demand for steady-state usage, while On-Demand instances handle variable demand without upfront commitment, minimizing total cost.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
10 On-Demand Instances running 24/7
Why it's wrong here
Running 10 On-Demand instances at full capacity 24/7 pays for idle capacity overnight — maximum cost with no optimization.
✗
10 Reserved Instances
Why it's wrong here
10 Reserved Instances commits to paying for 10 instances regardless of whether they're needed overnight.
✓
2 Reserved Instances + Auto Scaling with On-Demand for variable demand
Why this is correct
Reserved Instances cover the predictable 2-instance baseline at lowest cost, while Auto Scaling adds On-Demand capacity during business hours and removes it overnight.
Related concept
Read the scenario before looking for a memorised answer.
✗
10 Spot Instances with Auto Scaling
Why it's wrong here
Spot Instances are appropriate for interruptible workloads but are not reliable for business-critical applications requiring guaranteed availability.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may choose Spot Instances (Option D) for cost savings without recognizing their lack of reliability for a workload requiring consistent availability during business hours, or they may over-purchase Reserved Instances (Option B) assuming all capacity must be reserved, ignoring the variable nature of the workload.
Detailed technical explanation
How to think about this question
Reserved Instances provide a billing discount based on a commitment to a specific instance family, region, and tenancy, with Standard RIs offering up to 72% savings over On-Demand. Auto Scaling with On-Demand instances uses CloudWatch alarms and scaling policies to add or remove instances based on metrics like CPU utilization, ensuring the application scales to 10 instances during business hours and scales down to 2 at night. A real-world scenario might involve a retail website with predictable daytime traffic spikes; combining RIs for baseline and On-Demand for bursts avoids over-provisioning and reduces total cost of ownership.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CLF-C02 question in full detail.
Cloud Technology and Services — This question tests Cloud Technology and Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 2 Reserved Instances + Auto Scaling with On-Demand for variable demand — Option C is correct because it uses 2 Reserved Instances to cover the baseline workload (overnight) at a lower cost, and Auto Scaling with On-Demand Instances to dynamically add capacity during business hours. Reserved Instances provide a significant discount over On-Demand for steady-state usage, while On-Demand instances handle variable demand without upfront commitment, minimizing total cost.
What should I do if I get this CLF-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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