What statement best describes modulation?

The correct choice accurately identifies modulation as the process of imposing intelligence onto a radio carrier signal. In radio communications, modulation involves varying one or more properties of the carrier wave—such as its amplitude, frequency, or phase—in order to encode information. This is essential for transmitting data over radio frequencies, as it allows the information signal (such as voice or data) to be effectively transferred over long distances by utilizing the properties of the carrier wave.

Other choices, while related to communication methods and processes, do not precisely capture the full concept of modulation. For example, changing mark-space to 1 and 0 refers more specifically to digital signaling rather than the broader concept of modulation. Adjusting frequency for optimal range pertains to tuning and communication efficiency rather than directly to modulation itself. Converting a carrier from low to high frequency does relate to frequency shifting but does not encompass the process of imposing information on the carrier, which is central to the definition of modulation. Thus, the most comprehensive and accurate representation of modulation is found in the correct statement.