Probability that an event A doesn't occur = 1-P(A) where P(A) is the probabiity that A occurs.

The probability that the target is hit by the FIRST shooter only = 0.9 * (1-0.7) * (1-0.5)=0.9*0.3*0.5=0.135

The probability that the target is hit by the SECOND shooter only = (1-0.9)*(0.7)*(1-0.5)=0.1*0.7*0.5=0.035

The probability that the target is hit by the THIRD shooter only =(1-0.9)*(1-0.7)*(0.5) = 0.1*0.3*0.5= 0.015

Hence the probability that the target is hit only by one = 0.135+0.035+0.015 = 0.185 (OPTION B)