According to the PMBOK® Guide, specifically within the Plan Human Resource Management process (now known as Plan Resource Management), the project manager must identify the types and quantities of resources needed to complete the project activities.

Activity Resource Requirements: This is a primary input to developing the human resource plan. These requirements are determined during the Estimate Activity Resources process. They identify the specific types of people, skills, and competencies needed for each work package or activity. By reviewing these requirements, the project manager can determine the total human resource needs of the project.

The Planning Logic: You cannot create a plan for how to manage your team until you know what kind of team you need. The " Activity Resource Requirements " provide the data on the " what " (e.g., 2 Java developers, 1 QA tester, 1 UI designer) which then allows you to create the " how " (the Human Resource Plan).

Other Key Inputs:

Enterprise Environmental Factors: The organization ' s culture, existing human resources, and marketplace conditions.

Organizational Process Assets: Templates for HR plans, lessons learned, and organizational charts.

Analysis of Other Options:

A. Project management plan: While the Human Resource Plan eventually becomes part of the Project Management Plan, the overall plan is not typically listed as a specific input to this individual subsidiary process in the same way the detailed resource requirements are.

C. Resource calendar: This is an output of the Acquire Resources process. It shows when specific resources are available. During the initial planning phase, you are defining requirements; you don ' t yet have the specific calendars for people who haven ' t been assigned yet.

D. Project staff assignments: This is an output of the Acquire Project Team process. It refers to the specific individuals assigned to the project. You cannot have assignments as an input to the plan that is designed to figure out how to get those assignments in the first place.