Which of the following is a responsibility of the First Officer?

The responsibility of the First Officer primarily revolves around assisting the captain and operating the aircraft. In a flight crew, the First Officer, also known as the co-pilot, plays a critical role in ensuring the safe operation of the aircraft. This includes managing various flight operations, conducting pre-flight checks, monitoring the aircraft systems during flight, and being prepared to take command if the captain is unable to continue. The First Officer works closely with the captain, providing support in decision-making and contributing to the overall flight management to guarantee a safe and efficient journey.

The other options reflect responsibilities that are not typical of a First Officer's role. For instance, replacing the captain on long-haul flights is not commonly a responsibility shared by the First Officer; rather, this role is specifically designated to the captain. Serving drinks to passengers and maintaining cabin conditions fall under the purview of flight attendants, whose responsibilities are focused on the comfort and safety of passengers in the cabin rather than the operation of the aircraft itself.