The answer is (b): the radiometer will spin with about the same speed at which it spins when the entire radiometer is illuminated, as seen in an mpeg video by clicking your mouse on the photograph below.
Actually, the maximum rotational speed of the radiometer is probably a bit smaller than with the entire thing illuminated, but it is very nearly the same.
The next Question of the Week delves into this line of experimentation a bit more.
