Answer:
vicarious modeling
Explanation:
Vicarious modelling is a process by which a person sees himself through another person performing an action that he would like to perform.
In this case, Joanna engaged in vicarious modelling: she wanted to lose weight but did not have enough confidence to do so. She saw her friend Sharon slim down, and Joanna pictured herself doing the same as Sharon. Through vicarious modelling, Joanna gained enough confidence to start losing weight herself.
