Correct choice is (a) True
To elaborate: The above statement is not necessarily true always but change of variables cartesian to polar form & changing into suitable form reduces the complexity in the evaluation of double integral however converse is also true for example ∬R x^2 y^3 dx dy is the given integral while solving there is no need of converting it into polar form since ∬P r^6 cos^2 θ sin^3 θ dr dθ is tedious to solve when R only consists of constants but ∬R x^2 y^3 dx dy can be solved easily by ordinary method of integration.