He was reasonable enough to behave according to the circumstances.

The correct answer is C. sensible.

A sensible person is able to think clearly and make good decisions. They are able to understand and respond to the situation they are in. In this case, the person was able to behave in a way that was appropriate for the situation.

The other options are not as accurate. A peaceful person is someone who is calm and relaxed. An attentive person is someone who pays attention to what is going on around them. A determined person is someone who is focused on achieving their goals.

In this case, the person’s behavior was not necessarily peaceful, attentive, or determined. They were simply behaving in a way that was sensible given the circumstances.