What is the function of the $eq operator in a MongoDB query?

The $eq operator in a MongoDB query is used to find documents that have a specific field equal to a designated value. When you use $eq, you are specifically telling MongoDB to return documents where the value of a given field exactly matches the value you specify in your query. This operator is particularly useful when you want to filter documents based on exact matches within your database.

For instance, if you are querying a collection of users and you want to find all users whose status is 'active', you would use the $eq operator to ensure that only those documents that match that specific value are retrieved. This operator is fundamental for performing equality checks during query operations.

The other options pertain to different functions that do not describe the primary use of $eq. For instance, while some operators are utilized for deletion, updating, or counting documents, the $eq operator is specifically about matching field values. Thus, the correct choice accurately encapsulates the role of the $eq operator in filtering documents within a query.