Answer: 100%
Tp=Propagation Delay= 300 x $10^-6$ sec
Tt=Transmission Delay= Length/BW =1000/$10^7$ =$10^-4$ sec
a=Tp/Tt= 3
N=7 (I think GateForum misprinted 127 instead of 7, as i have seen this same question at a diffrent place with same options but 7 instead of 127)
Utilization = N/(1+2a) = 7/(1+2x3) = 7/7 = 1 = 100%