Which modulation technique allows for multiple signals to be sent simultaneously over a single channel?

The correct answer is multiplexing because it is specifically designed to enable multiple signals to be transmitted simultaneously over a single communication channel. This technique effectively combines multiple input signals into one output signal, allowing efficient use of the available bandwidth.

In contrast, amplitude modulation, frequency modulation, and phase shift keying are all methods of varying a single carrier signal to encode information, but they do not inherently provide the capability to send multiple signals simultaneously over the same channel. While they can be used in conjunction with multiplexing to modulate individual signals, multiplexing itself is the core technique that enables the simultaneous transmission of multiple signals. This makes it a fundamental aspect of network communication for optimizing the use of available resources in channels.