Final answer:
The adjective in the provided sentence is 'best,' which describes the 'course of action' that Gwen decided to take.
Step-by-step explanation:
The adjective in the sentence 'Upon reflection, Gwen decided that the best course of action would be to step down as president of the board.' is best. An adjective modifies a noun or pronoun by giving more information about it. In this case, 'best' modifies 'course of action' by indicating that it is the most suitable or desirable among alternatives.