You are correct, the answer is C) lack of refrigerant. When a refrigeration unit is operating with a lack of refrigerant, it will cause the system to short-cycle. This is because the reduced amount of refrigerant in the system prevents the compressor from building up enough pressure to effectively complete the refrigeration cycle. As a result, the compressor will rapidly cycle on and off, leading to short-cycling behavior. The other options are incorrect because: A) Heavy loads would cause the compressor to run for longer periods, not short-cycle. B) Starting conditions may cause short-cycling initially, but the issue would be resolved once the system stabilizes. D) Hot gas defrost is a normal part of the refrigeration cycle and would not cause short-cycling on its own.