Which is not a factor that affects teenagers' ability to resist peer pressure and stand up for their own values?
how high or low their self-esteem is
the nature of their family background
the number of friends they have
the role models they look up to

The number of friends they have is the correct answer.

Explanation:

Peers refer to the individuals who are a part of a similar social group, and the term peer pressure signifies the effect that peers can exhibit on each other. Though peer pressure does not only mean in negative terms, the pressure illustrates that the procedure affects individuals to do things, which they might not opt to do.  

However, the term peer pressure is generally used when someone is talking about the behaviors, which are not regarded socially desirable or acceptable, like use of drugs or alcohol.  

The number of friends one has does not influence the tendency of teenagers to resist peer pressure and to stand up for their own values. Even if one has a single friend, he or she can be truly happy, so in real sense it does not affect whether he or she will be or would not be able to stand up to his or her peers. However, one's self-esteem, role model which one follows, and the family background can affect it a lot.