A unilateral contract is enforceable against:

A unilateral contract is a type of agreement where one party makes a promise to perform without an obligation on the other party to reciprocate. In the context of the choices provided, the optionor is the party who makes this unilateral promise, often in the context of an option contract in real estate.

In situations involving unilateral contracts, the optionor is legally bound to fulfill the terms of the contract if the other party, the optionee, decides to exercise their right to accept the offer. The unilateral contract essentially creates an enforceable obligation for the optionor once the optionee acknowledges their acceptance of the option, thus compelling the optionor to comply with the promised terms.

This enforceability stems from the nature of the unilateral contract itself, which highlights the commitment of the optionor to the terms laid out in the contract upon the optionee's acceptance. Conversely, the optionee does not have any obligations until they choose to exercise the option, which is why the optionee is not enforceable against under this type of contract until that point.

By understanding this dynamic, it is clear that the optionor is the party that remains bound by the agreement, making them the only party enforceable in a unilateral contract until the option is exercised by the option