Self-Inductance & Energy question from NEET 1994 (AIPMT)

Answer: (b)

1. Voltage across the inductor $V = L\,\dfrac{dI}{dt}$.
2. Rising part: constant positive slope, so $V$ is a constant positive value.
3. Falling part: constant negative slope, so $V$ is a constant negative value.
4. So $V$ vs $t$ is a rectangular (square) pulse: positive then negative. None of the given graphs shows this; the key gives (b).