The correct answer is B) thermal expansion valves. The primary purpose of the liquid line strainer in a refrigeration system is to prevent dirt and scale from entering and potentially clogging or damaging the sensitive thermal expansion valve. The thermal expansion valve is responsible for controlling the flow of refrigerant into the evaporator, and any blockage or disruption to its operation can significantly impact the system's efficiency and performance. Keeping the liquid line strainer clean helps ensure the proper functioning of the thermal expansion valve, which is a critical component in the refrigeration cycle. The other options, while important components of the refrigeration system, are not the primary purpose of the liquid line strainer. The evaporator coil, compressor, oil separator, condenser, and receiver are protected by the strainer, but the thermal expansion valve is the most sensitive and vulnerable component that the strainer is designed to safeguard.