C100DEV MongoDB Overview and Document Model Practice Question
A developer is reviewing the characteristics of MongoDB documents. Which two of the following statements are accurate? (Choose two.)
⚠ Common exam trap
The trap here is assuming that documents in a collection must have a uniform structure, but MongoDB's schema flexibility allows varied fields.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Each document must have a unique _id field, which can be of any BSON data type except an array.
The maximum BSON document size is 16 MB, and every document must have a unique _id field that can be any BSON type except an array. Documents in a collection need not share the same fields, they are stored in BSON not JSON, and field names cannot contain null characters.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Documents are stored in a human-readable JSON format on disk.
Why it's wrong here
MongoDB stores documents in BSON (Binary JSON), a binary-encoded format that is more efficient for storage, traversal, and encoding than plain JSON. While BSON is JSON-like, it includes additional data types and is not human-readable on disk. This binary format enables faster parsing and supports data types not available in JSON.
- ✓
Each document must have a unique _id field, which can be of any BSON data type except an array.
Why this is correct
Every document requires a unique _id field that serves as the primary key. The _id can be any BSON type except an array, allowing flexibility such as using ObjectId, strings, or numbers. This uniqueness ensures that each document can be uniquely identified within a collection, and MongoDB automatically creates an index on _id.
- ✗
Field names in a document can contain the null character.
Why it's wrong here
BSON field names cannot contain the null character (\0) because it is used as a string terminator in the BSON specification. Allowing null characters would break parsing. Additionally, field names should not start with a dollar sign ($) in most contexts, as it is reserved for operators. Thus, this statement is incorrect.
- ✗
Documents in the same collection must have the same set of fields.
Why it's wrong here
MongoDB collections are schema-flexible; documents within a collection can have different fields. This flexibility allows for evolving data models without migrations. Requiring identical fields would contradict MongoDB's document model, which is designed to accommodate polymorphic data. Therefore, this statement is inaccurate.
- ✓
The maximum size of a BSON document is 16 MB.
Why this is correct
MongoDB enforces a maximum BSON document size of 16 MB to prevent excessive memory usage and ensure efficient data transfer. This limit applies to individual documents, not collections. Understanding this constraint is crucial for data modeling, as it influences decisions to embed versus reference data when arrays or subdocuments could grow large.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official MongoDB exam blueprint
This C100DEV practice question is part of Courseiva's free MongoDB 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 C100DEV exam.