How must the decision height be monitored in a CAT II ILS operation?

In a Category II Instrument Landing System (ILS) operation, monitoring the decision height is crucial for maintaining safety during the approach and landing, especially given the reduced visibility conditions indicated in CAT II operations. The correct choice involves using the radar altimeter setting for a precise determination of the aircraft's height above the ground.

Radar altimeters provide the most accurate measurement of height above the terrain and are designed to function effectively in low visibility environments, making them ideal for CAT II operations. They utilize radio waves to gauge distance from the ground, offering real-time data that supports safe decision-making at critical phases of flight.

While altimeter output from the main altimeter and co-pilot assessments may be used in some operations, these methods are less reliable in the specific environment of a CAT II ILS, where precision is paramount. Visual references are also not applicable in low visibility operations since conditions may not allow for visual cues to confirm altitude, thus necessitating reliance on more accurate instruments like the radar altimeter.