Under what condition can the unique constraint be used during index creation?

The unique constraint during index creation in MongoDB can only be effectively applied when the fields being indexed all have unique values. This means that each value must be distinct across all documents within the collection; this is vital for ensuring that no two documents have the same value for the indexed field(s).

When a unique index is created, MongoDB enforces this uniqueness, thereby preventing the insertion or updating of documents that would create duplicates for the fields it covers. This promotes data integrity and allows the system to maintain consistent and predictable behaviors when accessing or querying indexed data.

For instance, if a unique index is created on a user email field, it ensures that no two users can have the same email address in the database. This characteristic of a unique index is what makes it so valuable for maintaining data quality, especially when fields like email, username, or serial numbers should not repeat across records.

In contrast, scenarios where the indexed fields have non-unique values or all share the same value do not meet the requirements for establishing a unique index, as they would not uphold the integrity that the unique constraint is meant to guarantee. Thus, option C highlights the essential condition under which the unique constraint becomes applicable.