The correct answer is A) thermal expansion valves. The primary purpose of the liquid line strainer in a refrigeration system is to prevent dirt and scale from entering the thermal expansion valve. The thermal expansion valve is a sensitive component that controls the flow of refrigerant into the evaporator coil, and any blockage or contamination in this valve can disrupt the proper operation of the refrigeration system. By installing a strainer upstream of the thermal expansion valve, it helps protect this critical component from potential damage or malfunction caused by debris in the refrigerant. The other options (B, C, and D) are not the primary purpose of the liquid line strainer, as it is not primarily intended to protect the condenser, receiver, compressor, oil separator, or evaporator coil piping from dirt and scale.