Atotal of 150 new recruits join an army training course. after one month, 30 have dropped out. what is the percentage decrease in the number of trainees after one month

So what will it be a percentage of? it will be the percentage of the original recruits, of the original 150.

for this, we need to divide the number of recruits that left over all recruits, and multiply by 100%:

so the number of trainees decreased by 20 %

Originally there were 150 recruits 
30 dropped out 
the percentage of decrease would be 30/150
which can be simplified to 1/5 
1/5 as a percentage is 20%

None of the options you showed are it.