Which of the following agreements must be in writing to be enforceable?

A net listing agreement must be in writing to be enforceable because it involves a real estate transaction where the agent's commission is contingent upon the sales price exceeding a specified amount. Due to the complexities and potential for disputes that can arise from such arrangements, written contracts provide clarity and legal protections for all parties involved. In real estate law, certain types of agreements are required to be in writing to ensure enforceability under the Statute of Frauds. This requirement helps to ensure that all terms are clear and agreed upon, preventing misunderstandings.

While other options like lease agreements, exclusive right-to-sell agreements, and purchase agreements may also have their own requirements for written documentation in certain circumstances, the emphasis on the net listing agreement specifically highlights the necessity of a written form in order to protect the interests of the agent and the owner, given the nature of the commission structure involved.