The correct answer is A) servicing a navigational aid. The reasoning is that vessels servicing navigational aids are exempt from complying with traffic separation schemes to the extent necessary to carry out their work. This is because their primary purpose is to maintain and service critical navigational infrastructure, which takes precedence over strict adherence to traffic separation rules. The other options are incorrect because: B) a vessel engaged in fishing is generally required to comply with traffic separation schemes, C) a vessel on pilotage duty would also need to follow the traffic separation rules, and D) a vessel towing another is not exempt from the traffic separation scheme requirements.