According to the PMBOK® Guide for the Plan Cost Management process, the Project Management Plan is a primary input. To develop a cost management plan, the project manager must review other components of the overarching management plan to ensure consistency and alignment.

The specific components of the Project Management Plan used as inputs include:

Health and Safety Management Plan: Provides information regarding safety requirements that may impact costs.

Quality Management Plan: Outlines the quality levels and standards that will require specific funding and resource allocation.

Project Life Cycle Description: Establishes the phases the project will go through, which dictates how costs will be estimated, tracked, and controlled.

Development Approach: Defines whether the project uses a predictive, adaptive, or hybrid approach, which significantly influences how the cost management plan is structured.

Analysis of other options:

A. Expert Judgment: This is a Tool and Technique, not an input. It is used to process the inputs to create the plan.

B. Lessons Learned: While past information is helpful, the formal input from the organizational level is categorized as Organizational Process Assets (OPAs). A " Lessons Learned Register " is usually an output of the Manage Project Knowledge process and an input to later planning phases, but the Project Management Plan is the foundational document required here.

C. Cost Estimates: These are an output of the Estimate Costs process. You cannot have formal cost estimates before you have created the Cost Management Plan, which defines the " how-to " for estimating those costs.

As per PMI standards, the Plan Cost Management process occurs early in the planning phase to establish the policies, procedures, and documentation for planning, managing, expending, and controlling project costs. Therefore, it relies on the high-level framework already established in the Project Management Plan.