The correct answer is D. All of the above.
Updating a project plan can result in a change of critical path, a decrease in project completion time, or an increase in project completion time.
The critical path is the sequence of tasks that must be completed on time in order to complete the project on time. If a task on the critical path is delayed, the entire project will be delayed. Updating a project plan can change the critical path by adding or removing tasks, or by changing the duration of tasks.
The project completion time is the total amount of time it takes to complete the project. Updating a project plan can decrease the project completion time by identifying and eliminating tasks that are not necessary, or by improving the efficiency of tasks. Updating a project plan can also increase the project completion time by adding new tasks, or by extending the duration of tasks.
In general, updating a project plan should be done carefully and with consideration for the potential impact on the critical path and project completion time.