Which of the following key constraints is required for functioning of foreign key in the context of relational databases?

1. AUnique key
2. BPrimary key
3. CCandidate key
4. DCheck key

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Correct answer: B. Primary key

Explanation

A foreign key normally references the primary key of the parent table.

Note: Strictly, a foreign key may reference any candidate (unique) key; the official key is primary key.