A project is struggling because additional requirements keep being requested that prevent the team from focusing on the purpose of the project. This could have been avoided by using:

Quality metrics define standards for deliverables but do not prevent new requirements from being requested outside the original plan.

A budget allocates financial resources but does not define the project's work or prevent changes to its scope.

A Work Breakdown Structure (WBS) decomposes the project's work into smaller, manageable components but is based on an already defined scope, not a tool to prevent scope creep itself.

A scope statement formally documents the project scope, including project objectives, deliverables, requirements, and boundaries. By clearly defining what the project will deliver and what it will not, a scope statement helps prevent scope creep and ensures the team remains focused on the agreed-upon purpose.