Answer:
Positive reinforcement
Explanation:
Now, after you get the reward (bonus) for working extra hours (desired action), you will make an obvious association bewteen thse those two:
Desired action leads to a reward.
And since you (as any other human being) pursue your pleasure and your benefit, you will make extra hours more often to get the rewards. and that is positive reinforcement.
For learning more about positive reinforcement read about psychologist Burrhus Frederic Skinner.
Hope this helps!