Final answer:
The most likely cause of the described air conditioner issue is insufficient refrigerant charge, leading to lower evaporation pressure and temperature, resulting in suction line frosting.
Step-by-step explanation:
An air conditioner operating with low system pressures, an evaporator saturation temperature of 20 degrees Fahrenheit, and a suction line temperature of 24 degrees Fahrenheit with the suction line frosting indicates that the system is experiencing a cooling process with inadequate refrigerant levels. This condition is usually due to insufficient refrigerant charge, causing the remaining refrigerant to evaporate at a lower pressure and temperature than usual, hence the low evaporator saturation temperature and frosting along the suction line as the ambient air loses heat more rapidly to the colder refrigerant.