Which of the following systems is primarily used to monitor the pitch of a vessel?

The system primarily used to monitor the pitch of a vessel is the Vertical Reference Sensor. This type of sensor is specifically designed to detect and measure the vessel's orientation with respect to the vertical axis. By accurately determining pitch, roll, and heave, the Vertical Reference Sensor provides essential data for maintaining stability and ensuring precise dynamic positioning operations.

In dynamic positioning systems, understanding a vessel's pitch is crucial for effective maneuvering and maintaining the vessel's position in challenging maritime conditions. The Vertical Reference Sensor allows for real-time adjustments, enabling the DP system to respond to changes in environmental factors such as waves and wind.

In contrast, the other systems listed serve different purposes. Acoustic Position Reference systems are used for positioning by measuring the distance to an underwater transponder, while Differential GPS provides accurate positioning data based on satellite signals. The Gyrocompass, meanwhile, helps in determining the vessel's heading but does not directly measure pitch.