ANS:D - capillary tube.

A capillary tube is a narrow, small-diameter tube typically made of copper or another suitable material used as an expansion device in refrigeration and air conditioning systems. It serves to regulate the flow of refrigerant from the high-pressure side (condenser) to the low-pressure side (evaporator) of the system. The capillary tube works based on the principle of pressure drop along its length due to its small diameter and length. As the high-pressure liquid refrigerant enters the capillary tube, it encounters resistance due to its small diameter. This resistance creates a pressure drop, causing the refrigerant to expand and evaporate partially as it flows through the tube. The length and diameter of the capillary tube are carefully chosen to achieve the desired pressure drop and control the flow rate of refrigerant into the evaporator. The capillary tube helps maintain a pressure difference between the high and low-pressure sides of the system, facilitating the phase change of the refrigerant from liquid to vapor in the evaporator, which is essential for cooling. Capillary tubes are simple and cost-effective expansion devices commonly used in small refrigeration systems, such as window air conditioners, refrigerators, and freezers. However, they may not provide as precise control over the refrigerant flow as other expansion devices like thermostatic expansion valves. Nonetheless, capillary tubes are suitable for applications where simplicity, reliability, and low cost are priorities.