According to the PMBOK® Guide, a Baseline (Scope, Schedule, or Cost) is the approved version of a project plan. It can only be changed through formal Change Control procedures and is used as a basis for comparison to actual results.

Approved Change Requests: When a change request is formally approved through the Perform Integrated Change Control process, and that change affects the project ' s scope, schedule, or cost, the corresponding baselines must be updated. This ensures that the " yardstick " used to measure performance reflects the new, agreed-upon reality of the project.

The Baseline ' s Purpose: The baseline exists to track variances. If you changed the baseline every time a project was late or a risk occurred (Options A, C, and D), you would lose the ability to measure how far the project has drifted from the original plan.

Analysis of Other Options:

A. A project is behind schedule...: This is often referred to as " re-baselining to hide delays. " Baselines should not be updated simply because performance is poor; the baseline must remain to show the extent of the delay.

C. A risk occurs, resulting in a delay: When a risk occurs, it is handled using contingency reserves or workarounds. While it impacts the actual data, it does not automatically change the baseline unless a formal change request is approved to modify the project ' s end date.

D. Resource leaves with no replacement: This is a project constraint or issue. While it will likely cause a variance in the schedule and cost, the baseline remains the same so the project manager can report the negative impact of that resource loss against the original plan.