Final answer:
Eligibility for marketplace in mixed immigration status households is determined on a case-by-case basis.
Step-by-step explanation:
The best description of marketplace eligibility for members of mixed immigration status households is d) Eligibility is determined on a case-by-case basis. In mixed immigration status households, the eligibility of each member to access the marketplace is assessed individually, considering their specific immigration status. This means that some members may be eligible while others may not, depending on their immigration documentation.