How do you delete the default _id index?

The default _id index in MongoDB is a unique index created automatically for the _id field of every document in a collection. This index is crucial for the functioning of MongoDB because it ensures that each document has a unique identifier, which is fundamental for document retrieval and data integrity.

Option B, stating that you cannot delete this index, is correct because MongoDB enforces the _id field as a unique key for each document in the collection. The system depends on this index to ensure that no two documents can share the same _id value. Consequently, any attempt to manually drop or alter this default index would not be supported by MongoDB, as it is integral to the structure and operational mechanics of the database.

This characteristic of the _id index emphasizes the importance of having a unique identifier for documents in collections and helps maintain database consistency and performance. The other options are not valid given that they suggest ways to modify or delete the _id index, which goes against MongoDB's design principles.