See the explanation
The option in the question stated that Size of the solid particles involved.
Your concept is wrong. Never mind.
The size of the reactant become small, this means that they have been broken into powders....
Now in the powder,more atoms appear than the clumped reactant... Rate of reactions will certainly increase, because atoms can more easily react
See the image. The inner atoms in A can't react easily. But in B, almost all atoms can react. Hope u get your answer and had cleared your concept.