Question

Which of the following best explains why most chemical reactions proceed more quickly when the concentrations of reactants are increased?
a. The increased concentration increases the number of collisions between molecules.
b. The products of chemical reactions are more stable at higher concentrations.
c. At higher concentrations, reactant molecules move more quickly.
d. At higher concentrations, product molecules are able to catalyze the reaction.

Answer 1

A. 
Because you increase the concentration of the reactants, it means that there are more molecules of the reactants present in a smaller amount of space.

This being the case, there is a higher chance that these molecules will collide with enough energy to overcome the activation barrier.

The other ones are completely wrong, but if you would like an explanation as to why they are, just ask :)