The correct option is (b) 20

The explanation: Time of spraying/tree = \(\frac{spacing}{forward \, speed} = \frac{(application \, width)}{number \, of \, nozzles*discharge \, rate} \)

\(\frac{9*60}{3*1000} = \frac{18}{n*5} \)

n = \(\frac{18*3*1000}{9*60*5} \)

n = 20.