It determines how difficult it is to start the reaction.
Answer: B
Explanation
The activation energy is defined as the threshold energy required to the overcome the potential barrier among the reactants to form products by chemical reaction method.
So a chemical reaction can be occurred only when the external energy applied exceeds the activation energy of the reactants.
Thus the amount of activation energy quantifies the reactivity of the reactants.
This means the reactants which are highly reactivity and can start the chemical reaction easily will have less activation energy.
So the high magnitude of activation energy indicates the difficulty in starting of chemical reaction.
If the activation energy is high that means the reactants are less reactive so they require high energy to start the reaction.
Thus the activation energy affect the chemical reaction leading to measure its difficulty to start a reaction.