When does a license become irrevocable?

The correct choice is based on the understanding that a license in real estate can become irrevocable under certain conditions, particularly when substantial improvements have been made by the licensee. When a licensee invests in significant enhancements to a property or otherwise acts in reliance on the license—such as making improvements or developing the property—the law may treat the license as irrevocable to protect the licensee's investment. This runs parallel to the concept of equitable estoppel, which prevents the licensor from revoking the license once the licensee has made substantial efforts or investments based on the expectation that the license would remain in effect.

The other options focus on separate events that do not typically affect the revocability of a license. For instance, the termination of a lease does not inherently relate to the nature of a license unless connected by specific terms, and neither occupancy duration nor the death of the licensee is directly relevant to the irrevocability of the license. Therefore, substantial improvements directly tie to the concept of irrevocable licenses, solidifying the rationale for this response.