Quick Answer
The answer is AWS Lambda, as it is the service that directly responds to S3 upload events to trigger Rekognition automatically. When an image is uploaded to an S3 bucket, you configure an event notification that sends a payload to a Lambda function, which then calls the Rekognition API to perform object detection—this event-driven architecture processes only new uploads, eliminating the cost of scanning existing images. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your understanding of serverless integration patterns, often appearing as a distractor where CloudWatch Events (scheduled, not event-driven) or Step Functions (needs a trigger like Lambda) are incorrectly chosen. A common trap is selecting SNS, but remember SNS is a passive notification channel that requires a subscriber to act, whereas Lambda actively executes code. Memory tip: think “S3 uploads fire Lambda, which fires Rekognition”—a chain of event-driven triggers.
This MLS-C01 practice question tests your understanding of machine learning implementation and operations. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 is using Amazon Rekognition to detect objects in images stored in S3. They want to reduce costs by processing images only when they are uploaded. Which AWS service should be used to trigger Rekognition automatically?
Amazon CloudWatch Events
Why wrong: CloudWatch Events can't directly trigger Rekognition; needs Lambda.
Amazon Simple Notification Service (SNS)
Why wrong: SNS can notify but does not directly invoke Rekognition.
AWS Lambda
Lambda can be triggered by S3 event and call Rekognition.
AWS Step Functions
Why wrong: Step Functions can orchestrate but not directly triggered by S3 upload.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
AWS Lambda
Option C is correct because S3 events can trigger Lambda, which calls Rekognition API. Option A is incorrect because CloudWatch Events can trigger on a schedule, not directly on S3 uploads. Option B is incorrect because Step Functions can orchestrate but not directly triggered by S3 uploads without Lambda. Option D is incorrect because SNS is passive and needs a subscriber.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Amazon CloudWatch Events
Why it's wrong here
CloudWatch Events can't directly trigger Rekognition; needs Lambda.
Amazon Simple Notification Service (SNS)
Why it's wrong here
SNS can notify but does not directly invoke Rekognition.
AWS Lambda
Why this is correct
Lambda can be triggered by S3 event and call Rekognition.
Related concept
Read the scenario before looking for a memorised answer.
AWS Step Functions
Why it's wrong here
Step Functions can orchestrate but not directly triggered by S3 upload.
Common exam traps
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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FAQ
Machine Learning Implementation and Operations — This question tests Machine Learning Implementation and Operations — Read the scenario before looking for a memorised answer..
The correct answer is: AWS Lambda — Option C is correct because S3 events can trigger Lambda, which calls Rekognition API. Option A is incorrect because CloudWatch Events can trigger on a schedule, not directly on S3 uploads. Option B is incorrect because Step Functions can orchestrate but not directly triggered by S3 uploads without Lambda. Option D is incorrect because SNS is passive and needs a subscriber.
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 20, 2026
This MLS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the MLS-C01 exam.
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