Answer:
D. Molly is on a variable-ratio schedule of reinforcement.
Explanation:
Variable-ratio schedule: In psychology, the term variable-ratio schedule is a reinforcement schedule which is a part of the operant conditioning theory. The variable-ratio schedule is defined as a reinforcement schedule whereby a response is being reinforced after an uncountable number of responses. This schedule is considered to be a high and steady responding rate.
Example: Gambling.
In the question above, the given statement signifies that Molly is on a variable-ratio schedule of reinforcement.