Final answer:
The listing of a residential property would be terminated before its expiration date if the owner and agent mutually agree to cancel the agreement on April 15, based on the options provided.
Step-by-step explanation:
The event that would terminate the listing before May 2nd, in the context provided by the question, is 'C. On April 15, the owner and agent cancel the agreement.' According to standard real estate contracts and the termination clause provided, either party can cancel the agreement before its expiration date if both parties agree to do so. This is generally done through mutual consent and often involves signing a cancellation of listing agreement.