A promise becomes an enforceable contract if it has a consideration and it is enforceable. The most crucial factor is enforceability. In the given case Abner promises to pay for Claudia's trumpet if the representative of brass & woodwind musical instruments, inc does not pay. There is no written contract between the two parties, so the agreement is not enforceable. This promise shall be enforceable only when it in writing.
Hence, the correct answer is:
d. only if it is in writing.