Q.Multiple choice: An attribute or combination of attributes that uniquely identifies each tuple in a relation is called a ______.

This MCQ checks the definition of the key that uniquely identifies tuples. …

The correct option is (b) primary key.

A primary key is defined as an attribute (or combination of attributes) whose value uniquely identifies each tuple in a relation; it must be unique and never blank.

Why the other options are wrong:

• (a) foreign key — a foreign key refers to the primary key of another relation to create a link; it does not uniquely identify tuples in its own relation.
• (c) domain — a domain is the set of permitted values for an attribute (for example, 0 to 100 for marks); it does not identify tuples. …

Do not confuse a primary key with a foreign key: only the primary key identifies tuples within its own relation. A 'domain' is the set of allowed values, a …