Which of the following is the communication channel in case of radio communication?

The maximum distance between the transmitting and receiving antennas for satisfactory communication in line of sight mode is $57.6 \ km$. If the height of the receiving antenna is $80 \ m$,the height of the transmitting antenna is (Radius of earth $= 6.4 \times 10^6 \ m$) (in $m$)

$A$ 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 ..... .

The bandwidth of a video signal is approximately ........

The minimum size of an antenna for transmitting electromagnetic waves at $1500 \text{ MHz}$ is

The electron density of $E, F_1, F_2$ layers of the ionosphere is $2 \times 10^{11} \, m^{-3}$,$5 \times 10^{11} \, m^{-3}$,and $8 \times 10^{11} \, m^{-3}$ respectively. What is the ratio of critical frequencies for the reflection of radio waves?