Which component controls the water drain system?

The component that controls the water drain system is the water drain valve. This valve is specifically designed to manage the removal of water from a system, ensuring that any accumulation of water, which can lead to contamination or operational issues, is effectively drained.

While the other components mentioned may have their own specific functions, they do not directly control the water drain system. The fuel discharge valve is primarily used for managing fuel flow rather than water. The pilot valve assembly is typically involved in regulating pressure or flow in systems but does not specifically relate to draining water from a system. The auxiliary control valve might assist in various operations but is not specifically tasked with controlling the water drainage.

Therefore, the water drain valve is the correct answer, as it addresses the specific need for draining water, which is crucial for maintaining equipment efficiency and preventing damage.