Which of the following is an additional watchstander for maximum plant reliability (MPR)?

The role of the master helmsman is crucial for ensuring maximum plant reliability. As an additional watchstander, the master helmsman is responsible for overseeing and managing the navigation and control of the vessel. This position is vital, especially during critical operations, as the master helmsman ensures that the vessel's movements are performed safely and effectively. Their training and expertise directly contribute to the operational reliability of the plant, particularly when faced with potential challenges or emergencies.

The other roles mentioned—maintenance technician, external safety officer, and communications operator—while integral to the overall operation of the plant, do not specifically focus on navigation and control in the same way that the master helmsman does. Each of these positions has its own set of responsibilities that support the plant's functioning but do not serve the primary function of ensuring that navigational safety and plant operations run smoothly during critical watch periods.