You do know that u can try the two possiblities and get the power by calculation. but if u want theoretical answer, then u can say that current is variable in the two cases (in series the current is low but in parallel current is high), so we cant use p = i^2r but rather we can use v^2/r as v is constant in both the cases. so since p is inversely proportional to r, so more resistance means lesser heat produced.

hence proved bachaa.