Which of the following alkenes will react fastest with $\mathrm{H}_2$ under catalytic hydrogenation conditions?

During catalytic hydrogenation, the hydrogens are transferred from the catalyst to the same side of the double bond. Evidently, smaller the number of $R$ substituents, lesser is the steric hinderance and hence faster is the rate of hydrogenation. Thus, option (a) with two $R$ groups on the same side of the molecule is correct.