Reflection & Mirrors question from NEET 2013

Answer: (d)

1. Incidence $\theta$ at the first mirror, so the reflected ray makes $90^\circ - \theta$ with that mirror.
2. In the triangle formed by the two mirrors and the ray, the ray makes $180^\circ - 70^\circ - (90^\circ - \theta) = 20^\circ + \theta$ with the second mirror.
3. It leaves the second mirror at the same angle. To be parallel to the first mirror this angle must equal $70^\circ$.
4. $20^\circ + \theta = 70^\circ \Rightarrow \theta = 50^\circ$, so the answer is (d).