$A$ layer of the atmosphere that reflects medium-frequency radio waves and is ineffective during the night is

The process of superimposing a message signal on a high-frequency carrier wave is called:

$A$ $TV$ transmitting antenna is $128 \ m$ tall. If the receiving antenna is at the ground level,the maximum distance between them for satisfactory communication in line-of-sight mode is (radius of the earth $R_e = 6.4 \times 10^6 \ m$):

$A$ message signal of frequency $20 \, kHz$ and peak voltage of $20 \, V$ is used to modulate a carrier wave of frequency $1 \, MHz$ and peak voltage of $20 \, V$. The modulation index will be ..... .

This question has Statement-$1$ and Statement-$2$. Of the four choices given after the statements, choose the one that best describes the two statements.
Statement-$1$: Sky wave signals are used for long-distance radio communication. These signals are, in general, less stable than ground wave signals.
Statement-$2$: The state of the ionosphere varies from hour to hour, day to day, and season to season.

Which of the following is not a basic component of a communication system?