Overheating of the oil in a hydraulic system can be caused by __________.

Correct Answer: Option D — continuous leakage through the pressure relief valve

Continuous leakage through a pressure relief valve causes high-velocity fluid flow across the valve seat, converting hydraulic energy into heat. This constant energy dissipation significantly raises the bulk oil temperature, potentially degrading fluid viscosity and damaging seals, which necessitates immediate inspection of the relief valve's seating surfaces.