Answer:
d. variable-ratio
Step-by-step explanation:
Variable-ratio schedule: In psychology, the term "variable-ratio schedule" is described as a schedule of reinforcement that is considered as a part of the operant conditioning which was introduced by B. F. Skinner. It is referred to as a reinforcement schedule in which a specific response is being reinforced after an "unpredictable" number of particular responses. However, it creates a high yet steady rate of responding.
Example: Lottery games.
In the question above, the given statement represents a variable-ratio schedule.