Explanation

(c) Let resistors $A,\ B\ and\ C$ have equal resistance $R.$
Let $I$ be the total current then the current in resistor $A\ is\ I$ and in resistor $B\ and\ C$ are I/2.
So, heat produced in resistor $A$ is
$H_{A} = I^{2}R\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ldots(i)$
and heat produced in resistor $B$ is
$H_{B} = \left( \frac{I}{2} \right)^{2}\ R = \frac{I^{2}R}{4}\ \ \ \ \ \ \ \ \ \ldots(ii)$
and heat produced in resistor $C$ is
$H_{C} = \frac{I^{2}R}{4}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ldots(iii)$
Hence, it is clear that the heat produced will be maximum in $A.$