Final answer:
The answer is a principal of the firm.
Step-by-step explanation:
The answer to the question "Customer complaints must be handled by whom?" is 4) A principal of the firm. When a customer has a complaint, it is typically the responsibility of a principal, who is a high-ranking official or executive within the company, to handle and address the complaint. A principal has the authority and knowledge to resolve the issue and ensure that the customer's concerns are addressed appropriately.