Final answer:
Mark's irregular payments for his legal services represent a variable interval reinforcement schedule, which is based on unpredictable time intervals and encourages a steady level of performance.
Step-by-step explanation:
Mark, as a lawyer who owns his own practice and receives payments at irregular times, is experiencing a variable interval reinforcement schedule. This is because the reinforcement (payment) is based on unpredictable amounts of time.
This schedule is similar to the example provided where a restaurant manager receives a quality check at irregular intervals, providing a bonus for good performance if the check happens at that time.
Like with Manuel, Mark's irregular payment schedule would encourage him to maintain a consistent level of professional service, because he cannot predict when the next payment will come.