Treat it in english

let's tackle these.

Here are the answers to the remaining questions:

21) The correct answer is A).
Modulation is the process of varying one or more properties of a periodic waveform, such as an carrier signal, with a modulating signal that contains information to be transmitted.

22) The correct answer is C).
Multiplexing is a technique that allows multiple data streams to be transmitted over a single communication channel simultaneously. This increases the efficiency of the communication medium.

23) The correct answer is A).
Frequency Division Multiplexing (FDM) divides the total bandwidth of a channel into several non-overlapping frequency bands, with each band allocated to a different signal.