To send digital information, we must devise analog signals to represent bits. The process of converting between bits and signals that represent them is called digital modulation. With digital modulation:

A.the most straightforward form of digital modulation is to use a positive voltage to represent a 1 and a negative voltage to represent a 0, a scheme called NRZ (Non-Return-to-Zero)

B.schemes that directly convert bits into signals (occupying frequencies from 0 up to a maximum) result in passband transmission

C.it is possible to share a channel by multiple signals, this scheme is called baseband transmission

D.all of the above are true

Digital modulation is defined as modulation of carrier signal using discrete signals. The digital signal to be transmitted is first converted to analog signal using Digital-to-Analog converter. At the receiving end, the analog signal is converted back to digital signal using Analog-to-Digital converter.

Options:

B. schemes that directly convert bits into signals (occupying frequencies from 0 up to a maximum) result in passband transmission. (False)

Schemes that directly convert bits into signals result in baseband transmission, occupying frequencies from 0 up to a maximum.

C. it is possible to share a channel by multiple signals, this scheme is called baseband transmission (False)

It is possible to share a channel by multiple signals, sharing signals is called multiplexing.

A. NRZ (Non-Return-to-Zero) is a simple and straightforward digital modulation scheme. In NRZ, a positive voltage is represented by 1 and a negative voltage represents a 0. (True)

Therefore, A is the only correct answer.