I don't think this problem makes sense. The answer given is (a). Aren't the field lines parallel to the loop, what does rotation affect ? create atomic currents?
Answer
The magnetic field from the bar magnetic could be represented by magnetic field lines. This lines outside the magnet are bended between the poles and by this they cross the wire in its horizontal part. Since the magnet is rotating the field lines one by one crossing the horizontal part of the electrical circuit.
Now remember the Lorentz force $$\vec F = q \vec v \times \vec B $$
If a moving charge (an electron) goes through an external magnetic field (non-parallel to this magnetic field) then the charge gets deflected (perpendicular to the plane made by the electrons movement direction and the external magnetic field).
The Lorentz force is one of the three possibilities between the constituents magnetic field, relatively movement between this field and a charge and the reflection of the charge. Beside the Lorentz force the other phenomena are called induction of a current and induction of a magnetic field.
Due to this source the vector product from perpendicular vectors can be rewritten to $$ q \vec v = \dfrac {(\vec B \times \vec F)}{\|\vec {B}\|^2} $$ This is an equation that helps to understand that a current flows in the circuit and the answer is a).
For more details see on Wikipedia about the Homopolar generator:
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