Answer
Please see the work below.
Work Step by Step
(a) We know that the retarding effect is greatest on the solid disk. The holes in the slotted disk interrupt the flow of eddy currents, and it is the eddy currents that produce the opposing magnetic field responsible for the retarding effect.
(b) We know that the best explanation is option (I) -- that is, the solid disk experiences a greater retarding force because eddy currents in it flow freely and are not interrupted by the slots.