Most reactions speed up when
a. the temperature decreases
b. equilibrium is achieved
c. the concentration of the products increases
d. the reactant is in small pieces

For a reaction to occur, there should be mobility of ions in reactant side.
If the reactant is larger, its mobility will be lesser than that of smaller ones.
So reactants smaller in size have higher mobility which makes reaction faster.

Hence D is the correct option.

Hope this helps, have a great day/night ahead!