What does modulation in a radio system refer to?

Modulation in a radio system refers to the process of modifying a carrier wave's properties, such as its amplitude, frequency, or phase, to encode information for transmission. This technique allows information, such as voice or data, to be sent over the radio waves effectively. By changing the characteristics of the carrier wave based on the data being sent, modulation enables the transmission of complex signals over various distances and through diverse mediums.

The choice regarding the combination of signals for transmission incorrectly emphasizes the grouping of multiple signals rather than highlighting the specific process of altering a carrier signal to carry information. The other options also touch upon aspects of radio communications but do not accurately define modulation, which specifically focuses on how data is encoded onto the carrier wave rather than merely combining signals or converting waveforms.