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

The modulation index of an $FM$ carrier having a carrier swing of $200\, kHz$ and a modulating signal frequency of $10\, kHz$ is:

Television signals on earth cannot be received at distances greater than $100\, km$ from the transmission station. The reason behind this is that

$A$ bandwidth of $6 \, \text{MHz}$ is available for $A.M.$ transmission. If the maximum audio signal frequency used for modulating the carrier wave is not to exceed $6 \, \text{kHz}$,the number of stations that can be broadcasted within this band simultaneously without interfering with each other will be ..... .

In $AM$ modulation,a signal is modulated on a carrier wave such that maximum and minimum amplitude are found to be $6\,V$ and $2\,V$ respectively. The modulation index is $\dots \%$.

$A$ ground receiver station is receiving a signal at $5\, MHz$ transmitted from a ground transmitter at a height of $300\, m$,located at a distance of $100\, km$ from the receiver station. Given: Radius of Earth $R = 6.4 \times 10^6\, m$,$N_{\max}$ of ionosphere = $10^{12}\, m^{-3}$. Identify the mode of propagation.