8 khz, as voice frequency is from 300hz to 3400 hz . so max. is 3.4khz which is approx. taken as 4 khz. as we know sampling freuency should be atleast twice the highest frequency component so it comes out to be 2 time of 4 khz. i.e. 8 khz.
The voice signals have a bandwidth of 4Khz (300hz-3400hz signal bandwidth + 450*2hz guard bands= 4000hz) according to Nyquist criteria, the sampling rate must be at least double of this total bandwidth. thus sampling rate= 2*4Khz = 8Khz. this is the required sampling rate to avoid loss of voice signals in transmission.