The Nyquist theorem is important for voice connection because:

A.
It specifies that the digital sampling frequency is twice the maximum frequency used.

B.
It is the algorithm used for E&M signaling between PBXs.

C.
It helps prevent echo on voice circuits.

D.
It specifies that the digital sampling frequency equals the maximum frequency used.

E.
It specified that the digital sampling frequency is 4 times the maximum frequency used.

Explanation:
When sampling a signal , the sampling frequency must be greater than twice the bandwidth of the input signal in order to be able to reconstruct the original signal.