Answer:
D) variable-interval
Step-by-step explanation:
In the variable-interval schedule the reinforcement is provided after an unpredictable amount of time. This is a strong form of reinforcement that means that the action will be done irrespective of when the reinforcement will arrive.
This happens as the organism does not know when the reinforcement will arrive but knows that the reinforcement will arrive.
Hence, the question is referring to variable-interval.