From the description given in the situation on the question, it seems that what happens to Hallie is that (C) she had to adjust her career plan because her interests changed.
In the beginning, she wanted to become a Biologist, and that was why she earned her Bachelor’s degree and went on to study for 3 years as a Biologist. However, it seems that she has a change of heart and thus decided to become a Pediatrician instead, and this necessitates her to have to return to school to get an advanced degree.