First we have to determine the units of Force.
Force = mass * acceleration
Force = Newton = (kg-m)/(s^2)
Mass of each bullet = m
The number of bullets fired per seconds = n
The velocity of each bullet = v
We can say that the force applied on the vehicle can be expressed as = mnv
We can see that the units of force fits with the expression mnv. The answer is therefore letter B. mnv