What type of mission is characterized by immediate response?

Immediate response missions are specifically designed to address urgent situations that require quick action, and on-call missions fit this description perfectly. They are not pre-scheduled or planned in detail but are instead set to be executed as soon as the need arises, allowing for rapid deployment and responsiveness to emerging threats or crises.

In contrast, scheduled missions involve predetermined timetables for execution, and planned missions are those that have been thoroughly organized in advance with specific objectives, resources, and timelines. Rehearsed missions, while they may involve quick coordination, typically include practice elements and are not as tightly associated with immediate response needs. Thus, the nature of on-call missions distinctly aligns with the notion of immediate response, making it the correct choice.