The answer is (b): a (very small) negative voltage (about 0.2mV) will be sensed by the voltmeter. In fact, this voltage is present whether the magnet rotates CW or CCW, as seen in mpg videos by clicking on the directions. In addition, (c) and (d) are arguably true because the rod containing any free electrons is rotating but there is only a minimal field, the earth's field, which is also very small.
However, there is a small (about 0.1-0.3 mV) potential developed with this arrangement, probably due to contact potentials between the various materials in the system and the wires, similar to the potential developed in a thermocouple. The observed potential is the same for either rotational direction of the aluminum rod; it would likely be opposite in sign for opposite rotation direction if it were due to some sort of induction effect.
Question #218 is a follow-up to this seemingly null question.