This question fits the Process domain because it deals with document control, change control, and maintaining the integrity of project management plan versions. PMI describes configuration management as a disciplined process with methodology, systems, and procedures used to control change-related elements. The PMBOK errata also notes that updates to the project management plan occur through formal change control processes, which depend on reliable versioning and document control.
The missing latest version of the project management plan indicates a weakness in version control and document management, so the project manager should improve the configuration management process. That action addresses the root cause by strengthening storage, retrieval, baseline control, and change tracking.
Option A is weaker because quality assurance is broader and does not specifically solve version-control failures. Option C is useful for organizational learning but will not directly prevent future document loss.
Option D may be appropriate as an immediate administrative step, but it does not prevent recurrence by itself.
PMP-style reasoning favors fixing the management system that governs controlled documents, making configuration management the best answer.