What is the only default index that exists in each collection?

Each collection in MongoDB automatically includes a default index on the _id field. This index serves multiple critical purposes. First, it ensures that each document within a collection can be uniquely identified, as the _id field is a mandatory and unique identifier for documents. This uniqueness constraint is essential for many database operations, as it allows for reliable retrieval and manipulation of specific documents.

The _id index is created automatically by MongoDB when a document is inserted into the collection, which means there is no need for developers to manually create this index. It also enhances the performance of queries that involve the _id field, as indexed fields allow for quicker lookups.

Other options, like an index on the user ID field or the created date, are not guaranteed to exist by default since they depend on the specific structure and usage of the collection. Similarly, an index on the primary key is not the correct terminology in the context of MongoDB, where the term "primary key" is typically associated with relational databases, whereas MongoDB designates the _id field as a unique identifier.