Hi 

This is Ritika

I franckly say I am not sure this answer is right or wrong because it has been a year ago when I use to practice this type of questions.

But according to me 

Force =mass *acceleration

    F  =m*dv/dt

And as he fires n bullet so

F=n*m*dv/dt

According to Newton third law every action has equal and opposite reaction.

So reaction force per sec on the man will be mndv/dt .

Please do tell me my answer is right or wrong if wrong what is correct answer.

Thank you