During modulation, the transmitter maps digital information onto certain physical characteristics of a carrier signal so that the information can be transmitted over the air. The three classic carrier parameters that can be modified are amplitude, frequency, and phase. Therefore, options A, B, and D are correct.

Amplitude modulation changes the strength of the carrier wave. Frequency modulation changes the oscillation rate of the carrier. Phase modulation changes the phase angle of the carrier relative to a reference. Modern wireless communication systems often combine these parameters, especially amplitude and phase, in complex digital modulation schemes such as QPSK, 16-QAM, 64-QAM, or higher-order QAM used in WLAN standards. Option C, channel, is not a carrier parameter modified during modulation. A channel refers to a frequency resource or spectrum allocation used for communication, not a physical waveform characteristic being directly altered to encode bits. HCIA-Datacom introduces these basic wireless concepts to help learners understand how WLAN signals are transmitted, how data rate improvements are achieved, and why different modulation methods have different requirements for signal quality and interference resistance.