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In this question, option A is correct because the emf enduced in the rod will be because of the change in flux when the rod sweeps that area. magnetic field is uniform and the angular velocity of the rod is constant so the flux it is interacting with is constant and the enf produced will bt constant for half of the cycle. when the other side of the rod enters in, one side goes out so the direction of emf switches and it becomes a constant in opposite direction. you can ask further if you have any doubt...
why the direction of emf switches?
because as the rod rotates let first side A is in the field so it is positive and the other end is neutral. when side B enters now B is positive and A is neutral but you are studying emf from side A to B. so once it is positive and later is it negative