It says "the more reactants involved, the less likely for a fruitful collision." This is not talking about the amount of reactants, but about the amount of different reactants needed to come together AT ONCE. For that option you would need 2X and 2Y. That is a lot less likely than just X and Y.
I know, I know, that explanation was about as clear as mud. Let me try it another way.
In option a) X + Y --> Z
In option c) 2X + 2Y --> W + Z
Option a reactants have less variables than option c. Because there are more variables in option c, the desired product is less likely to come about. Hopefully that helps make it clear.