Which condition allows an estate to be defeated if a specific act happens afterward?

The correct choice is the condition subsequent, which refers to a situation in which an estate can be terminated or defeated if a specific act or circumstance occurs after the estate is created. This means that the property rights can continue indefinitely until a particular event takes place that leads to a loss of those rights.

For example, if someone holds a property "on the condition that no alcohol is consumed on the premises," the ownership will remain intact until that specific act – consuming alcohol – occurs, which would trigger the defeat of the estate. This illustrates how a condition subsequent operates.

In contrast, a condition precedent requires an event or action to occur before an estate can be created. Restrictive covenants are limitations placed on the use of the property that do not directly involve the termination of an estate. An easement by necessity refers to a right to use another person's land for a particular purpose, typically to access one's own land, and is unrelated to conditions that would defeat an estate.