In a situation where individuals act collectively, which term would best apply?

The term "en masse" is the most fitting in this context because it directly refers to the concept of individuals acting together as a large group or collective. In various scenarios, such as protests, movements, or collaborative efforts, individuals unite to take action that represents the whole group's interest or intention. This term captures the essence of collective behavior, highlighting the unity and solidarity among the participants.

The other options do not convey the same sense of collective action. Acting "on an individual basis" suggests separate or isolated actions, which is the opposite of collective behavior. "Separately" explicitly indicates that individuals are acting apart from one another, while "gradually" refers to the pace of action rather than the collective nature of it. Thus, "en masse" is the appropriate choice for describing a situation characterized by collective action.