Final answer:
A restricted lubricating oil return passage in the compressor of an air brake system can cause a high discharge of oil from the supply reservoir drain valve.
Step-by-step explanation:
The condition that could cause a high discharge of oil from the supply reservoir drain valve in an air brake system is c) A restricted lubricating oil return passage in the compressor. This restriction can lead to oil being pushed back into the air system, causing excessive oil to exit the drain valve. Both a governor setting too high and too low affect the air pressure levels rather than oil discharge, and a slipping compressor drive belt would likely cause a loss of air pressure rather than excessive oil discharge.