Which of the following is NOT a type of radio transmission mode?

Waveform modulation (WM) is not recognized as a standard category of radio transmission modes, which is why it is the correct answer to this question. In radio communications, transmission modes are categorized into established types that are widely understood and utilized. Amplitude modulation (AM) and frequency modulation (FM) are well-known methods that vary the amplitude and frequency of the carrier wave, respectively, to encode information. Pulse modulation (PM) is another recognized technique that encodes information in the form of pulses, typically used in digital communications.

Waveform modulation, however, does not correspond to a specific modulation method used in radio transmission, which is why it stands out among the options given. Understanding the different modulation techniques and their applications is essential for effective communication system design and operation.