Which of the following key constraints is required for functioning of foreign…

Answer: Primary key

Explanation: A primary key uniquely identifies each row in the referenced table and provides the required identity that foreign keys point to. By referencing a primary key, a foreign key enforces referential integrity: every foreign key value must match an existing primary key value.

• Primary key: A chosen candidate key that guarantees uniqueness and (typically) NOT NULL, and is the usual target for foreign key references.

• Foreign key: A column in one table whose values must match values of the referenced primary key in another table, ensuring referential integrity.

• Candidate key and unique key: A candidate key is any column(s) that can uniquely identify rows; one candidate is designated as the primary key. A unique constraint also ensures uniqueness, and some DBMS allow foreign keys to reference a unique column, but the conventional and standard reference target is the primary key.

• CHECK constraint: CHECK enforces a condition on column values and does not provide uniqueness or identity; it is unrelated to establishing foreign key relationships.

Key takeaway: Primary key is the required and standard target for foreign keys because it uniquely identifies rows and enforces referential integrity. Note the practical nuance that some systems allow referencing a unique key as well.