Final answer:
The enforceability of a verbal promise to assume another's debt usually requires a written agreement under the Statute of Frauds, unless the main purpose of the promise is for the personal benefit of the promisor. In this case, the promise by Pumps Inc. must primarily serve its own benefit to be enforceable without writing.
Step-by-step explanation:
In the scenario where Pumps Inc. agrees to assume a debt of Quality Parts Company to Reliable Finance LP without a written agreement, the enforceability of the promise rests upon the Statute of Frauds. For most promises to pay another person's debt, the Statute of Frauds requires such an agreement to be in writing to be enforceable. There is, however, an exception to this rule when the main purpose or leading object of the promisor (in this case, Pumps Inc.) is to secure a personal benefit. If the promise by Pumps Inc. to pay the debt is primarily for its own benefit, then the agreement may be enforceable even though it is not in writing. Based on the information given, since the primary benefit cannot be determined from the provided options, to be enforceable, the promise must be for the benefit of Pumps Inc. itself. This is known as the Main Purpose Rule or the Leading Object Rule, which is an exception to the Statute of Frauds.