In the circuit shown, the resistance r is a variable resistance. If for $r=f R$, the heat generation in r is maximum then the value of f is :

Question

$\begin{array}{1 1}(1) \large\frac{1}{4} \\ (2) \large\frac{1}{2} \\ (3) \large\frac{3}{4} \\ (4) 1 \end{array}$