A positive return on investment for higher education. ---
happens when you get a really good scholarship and discounts on tuition
happens when the costs of higher education are less than you expected
O is when you land a job after graduating
is when your earnings potential is higher than the cost of your education

is when your earnings potential is higher than the cost of your education

Explanation:

A positive return on investment on a particular thing occurs when the benefit derived is more than the cost that was incurred while in a scenario whereby the cost is more than the benefit, a negative return on investment took place.

In this case, a positive return on investment for higher education occurs when your earnings potential is higher than the cost of your education.