Reactants are the substances that undergo a chemical change during a reaction and are present before the reaction occurs. So, the correct response would be "substances that are formed by reactions." However, please note that this statement is actually inaccurate. The accurate definition of reactants would be "substances that are present before the reaction and that are consumed in the course of the reaction."
If you are looking for the best choice among the provided options, it would closely relate to "substances that are present before the reaction." However, based on the given answers, the closest correct answer that matches your question about reactants is not actually listed. If you have the correct definitions, that would be ideal.