The document that describes how and when the benefits of the project will be delivered and describes the mechanisms that should be in place to measure those benefits is

(A) Project business case
(B) Project benefits management plan
(C) Project business documents
(D) None of these

The answer is: (B) Project benefits management plan
The project benefits management plan is the document that describes how and when the benefits of the project will be delivered and describes the mechanisms that should be in place to measure those benefits.