The correct answer is B) lack of refrigerant. A refrigeration unit will tend to short cycle when there is a lack of refrigerant in the system. Short cycling occurs when the compressor turns on and off rapidly, rather than running for a longer, more efficient cycle. This is because insufficient refrigerant in the system reduces the unit's ability to absorb heat, causing the compressor to overshoot the temperature setpoint and shut off prematurely. The other options are incorrect because: A) heavy loads would cause the compressor to run longer, not short cycle; C) during hot gas defrost the compressor is intentionally turned off; and D) starting conditions may cause short cycling initially, but not as an ongoing issue.