Which theory best explains how Dean learned to brush his teeth by watching his sister?

The social learning theory is the best explanation for how Dean learned to brush his teeth by observing his sister. This theory, developed by Albert Bandura, emphasizes that people can learn behaviors through observing others, rather than through direct experience alone.

In Dean's case, he likely watched his sister perform the action of brushing her teeth, which involved not just the physical act itself, but also the rationale and context surrounding it, like the routines and consequences associated with good dental hygiene. This process of imitation can occur through a model – in this case, Dean's sister – which highlights why social learning is so powerful.

Furthermore, social learning theory underscores the importance of attention, retention, reproduction, and motivation in the learning process. Dean would need to pay attention to his sister’s behavior, remember the steps involved in brushing his teeth, be able to reproduce the action, and ultimately be motivated to brush his teeth himself, perhaps because he wants to emulate his sister or receive similar positive reinforcement.