Which of the following is NOT a project management process group?

Correct answer: Defining

Option C is correct because 'Defining' is not one of the five process groups recognized by PMI's PMBOK Guide; the five official process groups are Initiating, Planning, Executing, Monitoring and Controlling, and Closing. Option A is incorrect because Monitoring and Controlling is a legitimate process group that tracks, reviews, and regulates project progress and performance. Option B is incorrect because Executing is a valid process group encompassing the work performed to satisfy project requirements. Option D is incorrect because Planning is a recognized process group that establishes the project scope, objectives, and course of action.