Inductors & DC Transients (Mixed) question from NEET 2012 (AIPMT, Prelims)

Answer: (d)

1. Voltage across the coil $V = L\frac{dI}{dt}$, i.e. $L \times$ slope of the $I$-$t$ graph.
2. From 0 to $T/2$ the slope is constant and positive, so $V$ is a constant positive value.
3. From $T/2$ to $T$ the slope is constant and negative (same size), so $V$ is the same constant value with negative sign.
4. So the answer is (d), the square wave.