The word "underscores" in this context means to emphasize or highlight the importance or validity of something.

It suggests that the reaction being referred to reinforces or strengthens the point that was being made.

Similarly, the word "emphasizes" means to give special importance or prominence to something, drawing attention to its significance. In this sentence, the reaction is seen as reinforcing or emphasizing the point that was previously made.

Therefore, "emphasizes" is the option that best captures the meaning of "underscores" in this sentence.