Which is an unfair practice in motor vehicle sales advertising?

Answer: C — Using misleading representations

Using misleading representations is considered an unfair practice in motor vehicle sales advertising because it can deceive customers into believing something that is not true or accurate. Publishing a sale price and listing financing options are common, legitimate marketing techniques in the industry. Providing free car washes, while potentially enticing to customers, is not inherently unethical or misleading. Therefore, C is the only option that represents an unfair practice in motor vehicle sales advertising.