Prof. Mandal walks to the market and comes back in an auto. It takes him 90 minutes to make the round trip. If he takes an auto both ways it takes him 30 minutes. On a Sunday, he decides to walk both ways. How long would it take him?

As it took Prof. Mandal 30 minutes for round trip to the market by auto, we can infer that the auto takes 15 minutes for one way trip.
When he walks one way and return by auto, it took him 90 minutes.
We know the auto would have taken 15 minutes while returning.
So, the professor takes (90 - 15) = 75 minutes one way while walking to the market.
So, for a round trip by walking, he will take 2 * 75 minutes = 150 minutes.

Hence, option D is the correct.