Which two subsystems make up the proximity sensing system (PSS)?

The correct response highlights the components integral to the proximity sensing system (PSS) by identifying the proximity sensing electronic unit (PSEU) and the proximity sensors as its foundational elements. The PSEU is responsible for processing the data collected by the proximity sensors, which detect the presence of nearby objects or aircraft. This interaction enables the PSS to ascertain and report relevant distances, ensuring safe operations, particularly during critical phases such as landing and approach.

The other options do not align with the specific architecture of the proximity sensing system. For example, while radar units and other sensing technologies are useful in aviation, they do not constitute part of the PSS framework. Only the combination of the proximity sensors with the PSEU empowers the system with accurate proximity awareness and functionality.