The correct answer is A) Chilled water system supply temperature. In an air conditioning system using chilled water circulation, the chilled water supply temperature should remain constant regardless of load changes. This is because the supply temperature is a critical parameter that determines the cooling capacity of the system. Maintaining a consistent supply temperature ensures that the system can effectively respond to changes in cooling demand and provide the necessary cooling effect. The other options are incorrect because they are not the primary parameters that should remain constant. The chilled water return temperature, compressor discharge temperature, and compressor suction pressure can all vary depending on the load and other system conditions, but the chilled water supply temperature is the key factor that must be kept stable for proper air temperature control.