The correct answer is E.
Let's locate the main argument in the passage:
Premises:
1. The French teacher charges more than her late predecessor. The French teacher lives out of town and has transportation expenses.
2. The transportation expenses are covered solely by the teacher's fees.
Conclusion: The French teacher is not taking advantage of her monopoly to earn more money.
Assumption E is actually a necessary assumption in order to arrive at the author's conclusion. The author must assume that the teacher is not receiving any other compensation for her transportation expenses apart from her fees to conclude that she is not taking advantage of her monopoly. Thus the assumption is correct.
We can double-check our answer by using the negative test:
Premises:
1. The French teacher charges more than her late predecessor.
2. The French teacher lives out of town and has transportation expenses.
3. The transportation expenses are NOT covered solely by the teacher's fees. OR: The teacher receives compensation for her transportation expenses regardless of her high fees.
Conclusion: The French teacher is not taking advantage of her monopoly to earn more money.
The negative test proves to us that the assumption is indeed correct because once used in its negative form, it immediately invalidates the conclusion.