Final answer:
A landlord cannot reject Mary Ellen's application solely based on her status as a single parent.
Step-by-step explanation:
According to fair housing laws, it would NOT be a legitimate reason to reject Mary Ellen's application based on her status as a single parent. Fair housing laws prohibit discrimination based on a person's family status, including being a single parent. Landlords cannot legally deny someone a lease solely because they are a single parent. The other options provided in the question, such as insufficient income, poor credit history, and inadequate rental references, are all legitimate reasons that a landlord can consider when reviewing a lease application.