Which piece of technology assists in determining motion and position at sea?

The Inertial Navigation System (INS) is the correct choice as it plays a crucial role in determining motion and position at sea. This advanced technology utilizes sensors and accelerometers to track the vessel's movements and position based on its initial known location. By continuously measuring changes in velocity and direction, an INS provides real-time data about the vessel's current position without the need for external signals, making it particularly useful in marine environments where satellite signals may be unreliable or unavailable.

Other technologies mentioned serve different purposes. An Emergency Beacon is primarily designed for distress signaling to alert search and rescue services, and it does not provide navigation or position information. A Depth Sounder measures the water depth beneath the vessel, which is essential for ensuring safe navigation in shallow waters but does not assist in determining the overall position or motion of the vessel. A Visual Rangefinder is used for measuring distances using optical technology, which is beneficial for visual navigation but lacks the comprehensive positional data that an INS provides.