Which of the following could be a common sign of a refrigerant leak?

Ice buildup on coils is a common sign of a refrigerant leak because refrigerant is responsible for absorbing heat from the coils and creating a cooling effect. When there is insufficient refrigerant due to a leak, the pressure in the system drops, which leads to a decrease in the temperature of the coils. As a result, moisture in the air can condense on the coils and freeze, causing ice buildup. This can disrupt the cooling process and indicate the need for maintenance or repair.

The other signs, while they may indicate issues within the refrigeration system, are not as directly linked to refrigerant leaks. Increased compressor noise can suggest mechanical problems or increased strain but doesn't specifically indicate a leak. Unpleasant odors might arise from mold or decay within the system but are not directly correlated with refrigerant levels. Frequent resets of the thermostat may be related to electrical issues or temperature control problems, rather than a refrigerant leak. However, ice buildup on coils is a clear and strong indicator of refrigerant deficiency due to leaks.