72.
The answer is: D
Explanation
The correct option is D: "On a transfer of property, an interest therein is created in favor of a person to take effect only on the happening or not happening of a specified uncertain event."
Option D describes a contingent interest, which is an interest that only becomes vested (i.e., certain and absolute) upon the occurrence or non-occurrence of a specified uncertain event.
In contrast, the other options (A, B, C, and E) all describe vested interests, where the interest is either created immediately (A), postponed but not contingent (B), reserved for another person (C), or passes to another upon the occurrence of certain events (E).