Concept:Arrange the given parts to form a logical and grammatically correct sentence.
Explanation:Part P "When I was a student" introduces a time clause and must come first.
Part R "of class X" specifies the class and naturally follows "student".
Part Q "I learnt swimming at the age of 15" gives the main action and fits after the time and class are set.
Part S "in a government school" provides the location and ends the sentence.
Starting with Q or S would be incorrect because they lack the introductory time clause.
Thus the correct order is P, R, Q, S.
This matches option (b) in the list.
Answer:b) P R Q S (Option B).