Answer:
C. a variable-ratio schedule
Step-by-step explanation:
Variable-ratio schedule: In psychology, the term "variable-ratio schedule is described as one of the different reinforcement schedules which are considered as one of the important parts in the operant conditioning theory and were proposed by B. F. Skinner. In a variable-ratio schedule, a specific response is being reinforced after an "unpredictable number" of particular responses. However, it is often considered as creating high rate yet steady responding.
Example: Lottery and gambling.
In the question above, the given statement represents a variable-ratio schedule.