A contract in which a licensee is employed by an owner to find a buyer...

Comprehensive and Detailed Explanation (150–250 words):

A listing contract is an agreement in which a property owner employs a broker to find a ready, willing, and able buyer on specified terms. In this case, the owner of a 20-unit apartment building hires a licensee for that purpose.

A (option contract): gives a buyer the right to purchase property within a set time, not relevant here.

C (property management contract): involves ongoing operation of rental property, not selling it.

D (contract of sale): is the purchase agreement between buyer and seller, not the employment of a broker.

Therefore, the agreement described is a listing contract.