According to the PMBOK® Guide (Project Management Body of Knowledge), specifically within the sections on the Role of the Project Manager and Project Communications Management:

Communicating (Option D): It is a well-established principle in the PMI framework that project managers spend the vast majority of their time—frequently cited as 75% to 90%—communicating. This includes formal and informal communication with the team, stakeholders, sponsors, and customers. Because a Project Manager acts as the central link between the strategy and the execution, their primary " tool " is the exchange of information to ensure alignment, resolve conflict, and manage expectations.

Estimating (Option A): This is a specific activity within the Project Cost and Project Schedule management areas. While critical during the planning phase and during change control, it is a task-oriented activity that does not consume the bulk of a Project Manager ' s daily schedule.

Scheduling (Option B): Developing and maintaining the project schedule is a core function, but in many modern project environments, much of the data entry and logic is handled by scheduling software or project coordinators. The Project Manager focuses more on the implications of the schedule, which requires communication.

Controlling (Option C): Controlling involves monitoring project performance and implementing changes. While it is a continuous process throughout the project life cycle, " controlling " is often executed through communication (meetings, reports, and negotiations).

In the PMI framework, Project Communications Management is often considered the " oil " that keeps the project engine running. A Project Manager who communicates effectively can often overcome technical or resource deficiencies, whereas a Project Manager with poor communication skills will likely struggle even with a perfect plan and unlimited resources. Success is heavily dependent on the ability to manage the Communications Management Plan effectively.