The answer is D: positive reinforcer.
Explanation for each option:
A. Negative punishment: This involves removing a desirable stimulus to decrease a behavior. For example, taking away a toy when a child misbehaves. This is not applicable here because the child is receiving something (a sticker) rather than having something taken away.
B. Negative reinforcer: This involves removing an unpleasant stimulus to increase a behavior. For example, stopping a loud noise when a child completes their homework. This is not applicable here because nothing is being removed; instead, something is being added.
C. Positive punishment: This involves adding an unpleasant stimulus to decrease a behavior. For example, scolding a child for not doing their homework. This is not applicable here because the goal is to increase the behavior of picking up toys, not decrease it.
D. Positive reinforcer: This involves adding a pleasant stimulus to increase a behavior. In this case, giving a child a sticker each time they pick up their toys is intended to encourage and increase the behavior of picking up toys. Therefore, this is the correct answer.