To answer this question we need to remember what activation energy is. Activation energy is the "barrier" that must be overcome for the reaction to start, in other words, all chemical reactions even those that release energy (exergonic) need a certain amount of energy so they can start and continue. With this in mind, we can proceed to analyze the available options.
a. move molecules together. This is not correct, as the molecule movement does not necessarily imply a reaction.
b. lost to the environment. This is not correct as the loss of energy receives another name.
c. required to start a reaction. This is correct as is the necessary energy to a reaction start.
d. released by a reaction. This is not correct as this is only characteristic of exergonic reactions.
e. available to do work. This is not correct as the available energy is another concept.