Final answer:
The rotating shaft seal is most likely to leak on an open compressor refrigeration unit after not being used for a while. The condenser is the air conditioner component responsible for converting refrigerant gas into liquid.
Step-by-step explanation:
On refrigeration units that use an open compressor, the component most likely to leak if the unit is not used for several months is the rotating shaft seal. This part is susceptible because it contains moving parts and can dry out when not in use, leading to leaks. In contrast, service valves and oil drain plugs are less likely to have this issue due to their static nature and tight seal.
The component of an air conditioner that converts gas into liquid is the condenser. The condenser's primary function is to facilitate heat transfer, which results in the condensation of the refrigerant gas back into liquid form.