The correct answer is biological anthropology, or physical anthropology. It is centered around human beings only, with respect to their common or different features that they either share or don't share with their related non-human primates and other ancestors. It deals with human evolution, the study of their behavior, reproduction, etc.
I think the passage is trying to tell the reader that they shouldn't lose faith in people/animals just because they are elderly/weak when they are loyal to you because it will hurt their self-esteem.
