The document that defines the process for managing requirements revisions is the:

The change management plan is a subsidiary plan of the overall project management plan. Its specific purpose is to establish the process for managing and controlling changes. This includes how revisions to requirements are submitted, evaluated for impact, approved or rejected, and then implemented and communicated. It provides the formal framework for handling all modifications to agreed-upon requirements throughout the project lifecycle.

A. The project management plan is a high-level, composite document; the change management plan is the specific component that details the revision process.
C. The communications management plan defines how, when, and by whom project information will be administered and disseminated, not the process for changing requirements.
D. The scope management plan defines how scope is defined, validated, and controlled, but the specific process for handling change requests falls under the change management plan.

Project Management Institute. (2017). The PMI Guide to Business Analysis. Section 6.6.2.1, p. 210. This section explicitly lists the "Change control process" as a key component of the business analysis plan or a separate change management plan.

Project Management Institute. (2015). Business Analysis for Practitioners: A Practice Guide. Section 5.6, p. 100. This section, "Manage Changes to Requirements," describes the need for a defined process to handle changes, which is documented in the change management plan.