Assertion. A primary key is used to uniquely identify the rows in a data table.

Reason. A primary key is a field or attribute which has a unique value for each row or tuple.

(a)

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Explanation
A primary key is used to uniquely identify the rows in a data table. It is a set of one or more attributes that can uniquely identify tuples (rows) within the relation.