What word describes an action that is prompted by the occasion rather than being planned in advance?

The correct term for an action that is prompted by the occasion rather than being planned in advance is "impromptu." This word is often used to describe something that is done spontaneously or without prior preparation, such as an impromptu speech or an unexpected performance. It conveys the idea that the action is taken in response to a specific situation or moment, rather than being premeditated.

In contrast, the other options do not match this context. "Indeterminate" refers to something that is not clearly defined or decided, which does not capture the essence of spontaneity. "Irreverent" describes a lack of respect for things that are generally taken seriously, which does not relate to planning or spontaneity. "Immaculate" means perfectly clean or free from mistakes, which again does not fit the context of planning versus spontaneity. Thus, "impromptu" is the most appropriate choice to describe an action driven by the moment rather than pre-planned.