Correct answer: A stereo signal usually consists of two channels, a mono signal of one channel.
A mono signal consists of only one channel. This means that all audio information is transmitted via this one channel. As a result, even on a stereo monitoring system, the signals on both speakers are identical.
A stereo signal consists of two or more different channels that can contain different audio information. Depending on the level and the panning, the result is a wide, spacious sound.
Pure mono mixes have no stereo width, as left/right positioning with one channel is not possible. In a stereo mix, a mono signal can be positioned in the mix via panning.