One of your classmates, sally, is a hardworking student, serious about her classes, and conscientious about her grades. Sally is also involved, however, in volunteer activities and an extracurricular sport. Is sally displaying rational loading ... behavior?

I believe the answer is: Yes, she is using her time allocated among school, studying, sports and volunteer work in a manner she deems best.

A behaviour would be considered as 'rational' if it being done for the purpose/Benefit of the doer. It involves utilizing all of that person's time and resources in order to obtain optimal operation. People who display rational behaviour tend to much more likely to achieve their life goals compared to the people who don't.