Final answer:
Charles's oral promise to transfer assets upon divorce may be unenforceable without additional legal context or a written agreement. While the promise itself isn't a unilateral contract or an illusory promise, enforceability of oral agreements related to marriage typically requires formalities not described here. The correct option is b.
Step-by-step explanation:
The question concerns whether Charles's promise to Holly to transfer one-half of his assets to her in the event of a divorce is enforceable.
Under the law, prenuptial agreements or promises concerning the distribution of assets upon divorce are typically enforceable if they are made in writing and meet certain legal requirements, such as full disclosure and the absence of duress. Charles's promise, as described, is an oral agreement, and oral promises related to marriage and divorce often require additional formalities to be enforceable. Without additional context, such as evidence of a written agreement or a statute that allows for oral agreements in such circumstances, Charles's promise may be considered unenforceable due to lack of formality.
Additionally, this promise is not a unilateral contract because it would require consideration and acceptance to be considered a contract. It is also not an illusory promise because there is a clear commitment by Charles to transfer assets contingent upon a specific event (divorce). Therefore, without further legal context or a written agreement, the best answer is that the promise is unenforceable (b).