Answer:
D) They had a unilateral, express agreement.
Step-by-step explanation:
In a unilateral contract, the offeror makes an express promise without a reciprocal agreement from another party. The offeror's express promise of payment requires that the other party performs.
In this case, professor Debby made an express promise to pay $50 to anyone that mowed her yard, and Max performed the yard mowing, therefore he is entitled to payment.