The right answer is 1.5 milliliter
Ampicillin is a broad-spectrum beta-lactam antibiotic acting on Gram-positive bacteria and certain Gram-negative bacteria.
We have the concentration (250mg / ml) and the amount of the antibiotic to administer, so we have to look for the volume by the law:
C = n / V ==> V = n / C = 375 (mg) /250 (mg/ml) = 1.5ml
So you should give 1.5 milliliters of antibiotics every six hours for five days.