Final answer:
A buyer is most likely to invoke a defense of frustration of purpose when an unforeseen event undermines the original intent behind entering into the contract, making the purpose of the contract moot.
Step-by-step explanation:
The party most likely to invoke a defense of frustration of purpose would be A. A buyer. Frustration of purpose occurs when unexpected events undermine the reason for entering into a contract, thus the affected party's principal purpose is substantially frustrated. For example, a buyer who enters into a contract for the purchase of goods for a specific event, which is then canceled due to unforeseen circumstances, may claim frustration of purpose as the goods are no longer needed for their original intent. Sellers, lessors, and licensors are less likely to invoke this defense because their purpose, receiving payment, generally remains unaffected by the specific use the buyer had intended for the goods or property.