Final answer:
A registered representative should forward a complaint from a customer to a principal, upholding business integrity and ensuring formal recording of the issue and its resolution.
Step-by-step explanation:
When confronted with a letter of complaint from a well-acquainted customer, a registered representative should proceed professionally despite the easy nature of the resolution or their years of experience. The ethical and correct action to take is to forward the complaint to a principal or the appropriate department in charge of handling customer complaints. Although the representative might know how to solve the issue, it is important to ensure that there is a formal record of the complaint and its resolution process. This upholds the integrity of the business practices and ensures that the customer's concerns are addressed in a manner that is consistent with company policy. Therefore, the correct action is number 4: Forward the complaint to a principal.