How does activation energy affect a chemical reaction? a. it increases the energy of the reactants. b. it determines how difficult it is to start the reaction. c. it determines how much product can form. d. it decreases the energy of the products.

The correct answer is option B.

It determines how difficult it is to start the reaction.

Activation energy is defined as the minimum energy or the threshold energy required for the reactants to get converted to products. Thus it is the minimum energy required to start a chemical reaction. A higher activation energy means it is much more difficult to start a chemical reaction whereas a   lower activation energy means it is much easier to start a chemical reaction.  

the answer is symmetrical : )