To describe someone's agreement as acquiescing indicates:

Describing someone's agreement as acquiescing suggests that they are passively accepting an idea or proposal without strong enthusiasm or objection. This term is often used to imply a certain reluctance or lack of eagerness in the acceptance, indicating that the person goes along with something, perhaps because it is easier than opposing it. The nuance of "acquiescing" conveys a sense of compliance without an active endorsement, which aligns perfectly with the idea of passive acceptance.

Other options describe more active or emphatic responses, which do not fit with the connotation of acquiescing. A total rejection would indicate a refusal, loud opposition suggests a vocal disagreement, and enthusiastic endorsement implies a strong support that contrasts sharply with the passive nature of acquiescence.