Which condition must hold true for a field in 2NF?

For a field to be in Second Normal Form (2NF), it must not depend on a subset of the primary key. This is a crucial aspect of database normalization aimed at reducing redundancy and ensuring data integrity. In 2NF, every non-key attribute must be fully functionally dependent on the entire primary key, meaning that it should rely on the complete primary key rather than just part of it.

If a field were to depend only on a portion of the primary key, it would be violating the rules of 2NF as this could lead to unnecessary duplication of data. Ensuring that non-key fields depend on the whole primary key maintains a proper relational structure and eliminates anomalies during data operations such as insertion, deletion, or updating.

The other options do not accurately represent the requirements for a field to be in 2NF, as they address characteristics that are not related to functional dependencies in the context of normalization.