I believe that should be 24 mL, if it is mg, you have to memorize the conversion rate between g and mL (which I don't have off hand), so because 1 cubic centimeter = 1 mL:
12*24=288 cubic centimeters (hopefully you have this answer as one of the choices)
1) The antibiotic is being given by injection, therefore is in a liquid state.
2) They don't give you the mass density of the antibiotic solution.
3) Knowing the mass density of an antibiotic (esp. without telling you which one) off the top of your head is something they can't require you to do.
4) Water weighs 1 g/ml, and this they could expect you to know.
5) Therefore, you can conclude that they must expect you to use water's density.
6) Therefore, it's got to be a typo (they must've meant g not mg), so just substitute ml for mg and you're set.