According to the PMBOK® Guide, the process of auditing the quality requirements and the results from quality control measurements is the core definition of Manage Quality (historically and in some study guides referred to as Perform Quality Assurance).

Core Function: Quality Assurance (QA) is an execution-phase process that focuses on the processes used to create the deliverables. It ensures that the project team is following the defined organizational policies and project-specific quality management plan.

The Audit Mechanism: A key tool in this process is the Quality Audit. This is a structured, independent process to determine if project activities comply with organizational and project policies, processes, and procedures.

The Feedback Loop: QA uses the data generated by Quality Control (which measures the attributes of specific deliverables) to see if the overall process is working or if it needs improvement. If Quality Control shows frequent defects, Quality Assurance audits the process to find out why and implements corrective actions.

Comparison with Other Options:

Perform Quality Control (A): This process focuses on the deliverables. it monitors and records results of executing the quality activities to assess performance and ensure the project outputs are complete and correct.

Quality Metrics (B): This is an Output (attribute) of the Planning process, not a process itself. It describes a project or product attribute and how the control quality process will measure it.

Plan Quality (D): This is the Planning process where you identify which quality standards are relevant to the project and determine how to satisfy them.