option (D)
as we know in case of Go Back N ,N+1=2^n,where N is window size,n is number of bits for sequence number field
n=4 hence N=15
we know that link utilization or channel utilization or sender utilization=N/1+2a,where a=PT/TT
a=270*10^-3*1*10^6/125*8=270
efficiency=N/1+2a=15/1+2*270=.002772=2.772%(nearly 2.78%)