Which type of estate is subject to the occurrence or non-occurrence of a specified event?

The correct answer is the type of estate known as fee simple defeasible. This form of ownership is unique in that it is conditional and can be defeated or terminated upon the occurrence or non-occurrence of a specified event. Essentially, the ownership is granted with a specific condition attached. If this condition is met, the estate can revert back to the grantor or a third party, whereas failing to meet the condition does not affect the ownership.

In contrast, other estate types lack this conditional aspect. A life estate is tied to the lifetime of an individual, providing rights only until death. Fee simple, on the other hand, is an absolute form of ownership without conditions, giving the owner complete control over the property. An encumbrance is not an estate but rather a claim against the property that might affect its value or use, such as a mortgage or easement. Therefore, fee simple defeasible distinctly represents ownership dependent on specific events, underscoring its unique conditional nature.