According to the PMBOK® Guide (Project Management Body of Knowledge), specifically within the Project Procurement Management knowledge area and the Control Procurements process:

Inspection and Audits (Option B): This is a key tool and technique used to verify compliance in the seller’s work. While " Inspections " focus on the product or deliverable itself (physically verifying that the work meets requirements), " Audits " focus on the procurement process and the seller ' s adherence to the agreed-upon procedures. Both are conducted during the project ' s execution and monitoring phases to identify any non-compliance before the final handover.

Procurement Documents (Option A): These are considered Inputs to the Control Procurements process (such as the contract, statement of work, and bid documents). They provide the basis for the requirements but are not the " tool " used to perform the verification itself.

Estimate Budget (Option C): This is part of the Project Cost Management knowledge area (specifically the Determine Budget process). While costs are monitored during procurement, " estimating " the budget is a planning activity, not a compliance verification tool.

Risk Register (Option D): This is a project document (Input) that contains information on identified risks. While procurement involves significant risk, the register is used to track and monitor those risks, not to verify the physical compliance of a vendor ' s deliverables.

In the PMI framework, Control Procurements is the process of managing procurement relationships, monitoring contract performance, and making changes and corrections as appropriate. Inspections and audits are the primary mechanisms for the buyer to ensure the seller is fulfilling their contractual obligations regarding quality and process.