Final answer:
The least useful test in differentiating hemophilia A from hemophilia B is the platelet count.
Step-by-step explanation:
The least useful test in differentiating hemophilia A from hemophilia B is the platelet count (C). Hemophilia A is characterized by a deficiency of factor VIII, while hemophilia B is characterized by a deficiency of factor IX. The levels of these clotting factors can be measured using factor VIII (A) and factor IX (B) assays. Bleeding time (D) can also be used as a diagnostic tool for hemophilia, as individuals with hemophilia have prolonged bleeding times.