(c) Here: Current in the circuit
$(i) = 15\ mA = 15 \times 10^{- 3}A$
Resistance $R = 4000\ Volt$
Applied voltage in the circuit $= 240\ V$
At any instant, the $emf$ of the battery is equal to the sum of potential drop on the resistor and the $emf$ developed in the induction coil
Hence, $E = iR + L\frac{di}{dt}$
$240 = 15 \times 10^{- 3} \times 4000 + L\frac{di}{dt}$
Hence, $L\frac{di}{dt} = 240 - 60 = 180\ V$