Final answer:
The nurse should prohibit the family members from bringing raw sushi to the client with multiple myeloma and a low white blood cell count.
Step-by-step explanation:
For a client with multiple myeloma and a low white blood cell count, the nurse should prohibit the family members from bringing raw sushi to the client. Raw sushi is made with raw fish, which can contain harmful bacteria and parasites that can further weaken the client's immune system and increase the risk of infection. To ensure the client's safety, it is important to avoid any potential foodborne illnesses.