Excessive frost on the evaporator causes which of the following issues?

Excessive frost on the evaporator is detrimental to the refrigeration system primarily because it leads to reduced heat transfer. The evaporator's function is to absorb heat from the environment, allowing the refrigerant to change from a liquid to a gas as it absorbs this heat. When frost accumulates, it acts as an insulating barrier between the refrigerant and the airflow, which prevents effective heat exchange. This insulation means that the refrigerant cannot efficiently absorb heat, leading to lower overall system performance.

As for the other options, while reduced efficiency and increased superheat could be a consequence of decreased heat transfer, they are not the most direct issues caused by frost accumulation. High humidity refers to the amount of moisture in the air and is generally not a direct result of frost on the evaporator but rather an effect of the environment the system is operating in. Thus, the most accurate description of the problem caused by excessive frost is the reduction in heat transfer.