Which word refers to attempting to stir up public opinion?

The word "foment" is the correct choice because it specifically means to instigate or stir up, particularly in the context of public opinion or unrest. The root of the word relates to the idea of nurturing or promoting something, often in a way that leads to trouble or discord. Thus, "foment" highlights an active effort to provoke or incite reactions from a group or society, making it suitable for the context of stirring up public sentiment.

In contrast, "fracas" denotes a noisy disturbance or quarrel, which does not necessarily involve public opinion. "Felicitous" means well-suited or apt, often in a way that is pleasing or fortunate, and is unrelated to public sentiment. "Fractious," on the other hand, describes something that is irritable or quarrelsome, but again does not convey the idea of actively trying to stir up public opinion. Therefore, "foment" stands out as the word that directly addresses the act of inciting or stirring up sentiments among the public.