There are several words that can be used as synonyms for "make reasonable." Some of the most common ones include "rationalize," "justify," "legitimize," "validate," and "substantiate." Each of these words carries a slightly different connotation, but they all refer to the act of making something more reasonable or justifiable. For example, if someone is trying to make a questionable decision seem more reasonable, they might use one of these words to explain why it makes sense. Similarly, if someone is trying to convince someone else of an idea, they might use one of these words to add convincing weight to their argument.