Final answer:
Sexual orientation is the protected class under Massachusetts fair housing but not under federal fair housing. Although recent rulings offer some protection, there isn't clear nationwide legislation for housing discrimination.
Step-by-step explanation:
The protected class under the Massachusetts fair housing law but not under federal fair housing is c. sexual orientation. While federal civil rights laws don't explicitly include sexual orientation as a protected class for housing discrimination, Massachusetts law does. However, it's important to note that recent Supreme Court rulings have provided some federal protections against discrimination based on sexual orientation, particularly in employment. Additionally, the federal Department of Housing and Urban Development has suggested that refusing housing to transgender individuals may constitute sex discrimination, even though there isn't nationwide clarity on this aspect of the law.