Answer:
c. DRL
Step-by-step explanation:
Differential Reinforcement of Low rates Behavior (DRL) :
Unlike DRO which requires zero responding on the target behavior, reinforcement is delivered using DRL if the inappropriate behavior is at a tolerable or desirable level.
Reinforcement is provided either after a response that follows an interval of specific length or after a specified interval if the frequency of the behavior is below some established criterion.
Hence, Mary's parents are using DRL.