According to the PMBOK® Guide, specifically the Develop Schedule process, when a project is facing a time constraint (a fixed deadline), the project manager must employ Schedule Compression techniques to shorten the project duration without reducing the project scope.

Why Choice D is correct: Scheduling tasks in parallel is a technique known as Fast Tracking.

Fast Tracking: This involves performing activities that would normally be done in sequence (one after the other) in parallel for at least a portion of their duration. For example, starting to write the user manual while the software is still being coded.

Impact on Time: This directly reduces the total elapsed time of the project ' s critical path, helping to meet tight deadlines.

Risk Trade-off: While Fast Tracking saves time, it often increases risk and may lead to rework because tasks are being performed before the preceding task is 100% complete.

Analysis of other options:

A (Expand the scope): Expanding scope (Scope Creep) is the opposite of what should be done under a time constraint. More work typically requires more time, which would further jeopardize the deadline.

B (Schedule the tasks in sequence): Sequential scheduling is the " natural " flow of project work, but it is the least efficient way to save time. If a project is already under a time constraint, relying on a linear sequence is what leads to delays.

C (Increase quality review cycles): While quality is important, adding more review cycles consumes more time. Under a strict time constraint, the project manager might actually need to streamline processes rather than add extra steps, provided the Definition of Done is still met.

Key Concept: The Project Management Institute (PMI) emphasizes that a project manager must balance the " Triple Constraint " (Scope, Time, and Cost). When Time is fixed, Choice D (Fast Tracking) is the primary strategy used to compress the schedule by overlapping phases or activities, ensuring that the project reaches completion as quickly as possible without necessarily increasing the project ' s budget.