Slack time is the amount of time an activity can be delayed without delaying the overall project completion date. It gives project managers flexibility to reassign resources and manage schedule risk. A milestone is a significant point in time that marks completion of a deliverable, phase, or major event. Statements 1, 2, and 3 are therefore correct. Statement 4 is incorrect because a milestone normally has zero duration; it does not itself require time to complete, although the work leading to the milestone does. Internal audit may review project controls by evaluating schedules, critical paths, milestones, resource plans, and progress reporting. Therefore, Option C is correct.