Hannah's dad is providing positive reinforcement.
Positive reinforcement is a condition that involves the introduction of a reinforcing stimulus such as reward after a behavior which increases the likely recurrence of the same response in the future. Positive reinforcement strengthens a behavior or certain response. From the question, Hannah's dad was providing positive reinforcement by treating her to a chocolate sundae each time she brings home a straight-a report card, in order to motivate Hannah and to increase her chance of getting high grades again.