As we know that the sampling rate is two times of the highest frequency (Nyquist theorm)

Sampling rate=2 Nyquist

fs=8000hz/8khz

You must sample at 2 x the rate of the analog signal (2 x the analog signal frequency).

The Nyquist Therorem states that the lowest sampling rate has to be equil to or greather than 2 times the highest frequency. Therefore the sampling rate should be 400Hz or more.

Sampling rate is a defining characterstic of any digital signal. In other words, it refers to how frequently the analog signal is measured during the sampling process. Compact disks are recorded at a sampling rate of 44.1 kHz.

44.1kHz