Final answer:
Sharon's increase in excuse-making for turning in assignments late is due to modeling and vicarious reinforcement.
Step-by-step explanation:
The increase in Sharon's excuse-making for turning in assignments late is likely due to modeling and vicarious reinforcement. Sharon observed and learned from Kathy's behavior of successfully turning in late assignments with creative excuses. She then imitated Kathy's behavior and began using similar excuses to turn in her assignments late, expecting similar positive outcomes.