The key (C) is correct because the dose is 1 mL three times a day for 10 days which is 30 mL. The dose is 50 mg/mL so 50 mg x 30 mL is 1500 mg total in 10 days. If your stock concentration is 12 5mg/5 mL, then you will need 60 mL of that stock solution to provide the total dose for 10 days (i.e., 1500 mg x 5 mL divided by 125 mg = 60 mL) or 1500 mg/X mL = 125 mg/5 mL and solve for X
