Final answer:
The exclusivity for a listing agreement becomes effective upon signing the agreement, which is when the involved parties sign the contract.
Step-by-step explanation:
The exclusivity for a listing agreement becomes effective upon signing the agreement. As with most contractual agreements, the provisions within a listing agreement typically come into force at the point of execution, which is when both parties, the property owner and the real estate agent or broker, sign the document. This makes option (a) the correct choice. Options (b) 15 days after signing, (c) 30 days after signing, and (d) upon tenant approval are not standard trigger points for a listing agreement's exclusivity to commence.