Assertion (A) is TRUE: interchanging the cell and galvanometer in a Wheatstone bridge does not affect the balance condition — this follows from the reciprocity property of linear resistive networks.

Reason (R) is TRUE: the balance condition $\dfrac{P}{Q}=\dfrac{R}{S}$ is a relation purely between the four arm resistances, and since it doesn't involve which branch holds the cell vs. the galvanometer, the same ratio condition continues to give zero galvanometer current after interchanging — correctly explaining the assertion.

Answer: Both A and R are true, and R is the correct explanation of A.