Answer: must be in writing because it benefits McGregor.
Step-by-step explanation:
From the question, we are told that Country Farm Supply applies for a business loan from Farmers Credit Co-Op. We are also aware that Country Farm Supply owes McGregor money under another business deal, and that McGregor wants to help Country Farm Supply get the loan so that it will be able to stay in business to pay him back, McGregor promises Farmer's Credit Co-Op that he will repay the loan if Country Farm Supply does not.
Therefore to be enforceable, McGregor's promise must be in writing because it benefits him. The the deal is between Farmer's Credit Co-Op and McGregor, hence, the promise must be in writing because this will make it valid and also applicable for future purpose in case McGregor isn't able to repay the loan he took .