Final answer:
In project management, a plan is often not necessary for the contingency of Output, since it is typically the result of planning rather than an element that requires contingency planning.
Step-by-step explanation:
For which of the following contingencies will a plan not often be necessary? The options are a. Budget, b. Output, c. Duration, and d. Milestone. Planning is a crucial aspect of project management and operations. However, amongst the given options, a plan is usually not often necessary specifically for Output. The reason for this is that output refers to the results or the products of the operations, which are typically the consequences of the planning rather than being actively planned for in terms of contingencies. In contrast, a Budget requires a plan to ensure that resources are allocated efficiently, Duration needs planning to track timeline and meet deadlines, and a Milestone can serve as a checkpoint, and thus planning around it is critical to assess the progress of the project.