Final answer:
The situation where Sofia and Luke, along with Sofia's grandmother, misrepresent the nature of the down payment for a house is considered credit fraud.
Step-by-step explanation:
The situation described in the question is best classified as credit fraud. When Sofia's grandmother wrote a gift letter stating there are no repayment terms, but they have privately agreed upon repayment, it misleads the lender about the true nature of the down payment. Lenders require an accurate financial picture to assess risk correctly, and any misrepresentation in this process can constitute fraud.