Solution:

a. Before entering the compressor:
The refrigerant condition is a saturated liquid before entering the compressor. The primary purpose of the compressor is to increase the pressure of the refrigerant, not to change its phase. By ensuring that the refrigerant is in a saturated liquid state, it helps protect the compressor from potential damage that could occur if the compressor handles a two-phase mixture or vapor. Additionally, compressing a liquid is more efficient than compressing a vapor.

b. Before passing through the condenser:
The refrigerant condition before passing through the condenser is typically a high-pressure, high-temperature superheated vapor. After leaving the compressor, the refrigerant has been compressed and its temperature and pressure have increased. It enters the condenser as a superheated vapor to release heat and transition to a saturated liquid state.

c. After passing through the condenser:
After passing through the condenser, the refrigerant undergoes a phase change from a high-pressure, high-temperature superheated vapor to a saturated liquid. Heat is extracted from the refrigerant in the condenser, causing it to condense and change into a saturated liquid state. This saturated liquid refrigerant then moves on to the expansion valve.

d. After passing through the expansion valve:
After passing through the expansion valve, the high-pressure saturated liquid refrigerant experiences a significant pressure drop. This pressure drop results in adiabatic expansion, causing a decrease in temperature. As a result, a portion of the liquid refrigerant evaporates, forming a mixture of saturated liquid and vapor with lower pressure and temperature. This mixture then enters the evaporator to absorb heat and restart the cycle.

To summarize, the correct answer is:

a. Before entering the compressor, the refrigerant is in a saturated liquid state. This is important for compressor protection and efficiency in the vapor compression cycle.