Product requirements specify a functionality that depends upon expertise that is unavailable internally.

According to the PMBOK® Guide, specifically the Project Procurement Management knowledge area, the Plan Procurement Management process is the stage where the project team determines whether to acquire goods and services from outside the organization or to perform the work internally.

Make-or-Buy Analysis: This is a key Tool and Technique of the Plan Procurement Management process. It involves evaluating the costs, risks, and organizational capabilities associated with both options.

Trigger for Decision: In this scenario, the " functional requirement depending on unavailable expertise " is a direct trigger for a make-or-buy analysis. Since the expertise is unavailable internally, the analysis will likely lead to a " buy " decision to mitigate the risk of project failure.

Output: The primary output of this process is the Procurement Management Plan and the Make-or-Buy Decisions document, which outlines the strategy for engaging external vendors to provide the missing expertise.

Why other options are incorrect:

Option B (labeled incorrectly as B/Plan Risk Responses): While choosing to " buy " is a way to transfer risk, the specific formal process for generating a make-or-buy decision is Procurement Management, not Risk Response. Risk Response planning follows the decision to procure.

Option C (Conduct Procurements): This process occurs after the plan is finalized. It involves receiving seller responses, selecting a seller, and awarding a contract. You cannot conduct procurements until you have already made the " buy " decision in the planning phase.

Option D (Plan Risk Management): This process defines how to conduct risk management activities for a project. It does not address specific technical gaps or procurement decisions directly.