She accepted the prospect of her medical treatment with equanimity.

Equanimity means calmness or composure, especially in a difficult situation. The opposite would be a state of disturbance or restlessness. Tranquillity, composure, and placidity are all synonyms of equanimity, denoting calmness. Agitation, on the other hand, refers to a state of anxiety, nervous excitement, or disturbance, which is the direct antonym of equanimity. Therefore, 'agitation' is the correct answer.