According to the PMBOK® Guide, there is a distinct difference between how product scope and project scope are measured.

Project Scope Completion: The completion of the project scope is measured against the Project Management Plan. Specifically, it is measured against the Scope Baseline, which is a key component of the plan. The Scope Baseline consists of the approved version of the Project Scope Statement, the Work Breakdown Structure (WBS), and the WBS Dictionary.

Product Scope Completion: In contrast, the completion of the product scope is measured against the Product Requirements.

The Baseline Concept: Because the Project Management Plan contains the formal definitions of what work is included (and what is excluded), it serves as the " yardstick " for determining if the project has successfully completed its intended tasks. During the Validate Scope process, the actual work performed is compared to this plan to gain formal acceptance from the customer or sponsor.

Analysis of Other Options:

A. requirements documentation: This is used to measure the completion of the product scope (the features and functions that characterize a product, service, or result).

B. project scope statement: While the scope statement is part of the baseline, the PMBOK® Guide explicitly states that project scope completion is measured against the Project Management Plan as it contains the integrated baseline.

D. work performance measurements: These are used to assess the status and progress of project activities, but they are not the standard against which final completion is verified. Measurement against these helps identify variances, but the " finish line " is defined by the plan.