You are correct, the answer A) hydrostatic pressure of the expanding liquid is the correct answer. When a refrigerant container is overfilled, the liquid refrigerant expands as the temperature rises, generating high hydrostatic pressure within the container. This high pressure buildup can be extremely dangerous, as it increases the risk of the container rupturing or exploding. The other options are incorrect because they do not directly address the root cause of the pressure increase, which is the expansion of the liquid refrigerant. The discharge pressures from the recovery cylinder or compressor, as well as the vapor pressure of the refrigerant, are not the primary factors leading to the hazardous pressure buildup in an overfilled container.