For a bit-rate of 8 Kbps, the best possible values of the transmitted frequences in a coherent binary FSK system are

a) 16 KHz and 20 KHz

b) 20 KHz and 32 KHz

c) 20 KHz and 40 KHz

d) 32 KHz and 40 KHz

Ans.d) 32 KHz and 40 KHz

The values of frequencies used  for coherent PSK are

F_i=(n+1)/ T_b where n=1,2,etc

Given 1/T_b=8Kbps

So For n=3 ,F_i=(3+1)8=32Khz

and For n=4 ,F_i=(4+1)8=40Khz

fi = nc  + i /Tb

Given  Rb = 1 /Tb = 8 kbs

f1 = [ 3 + 1]8 = 32 KHz

f2 = [4 + 1 ] = 40 kHz