Which of the following modulated signals has the best noise tolerance?

The frequency range suitable for beyond-the-horizon communication using sky waves is:

The bit rate for a signal,which has a sampling rate of $8 \, kHz$ and where $16$ quantisation levels have been used,is ......... $bits/sec$.

$A$ $TV$ tower has a height of $100 \ m$. The average population density around the tower is $1000 \ \text{per} \ km^2$. The radius of the earth is $6.4 \times 10^6 \ m$. The population covered by the tower is:

$A$ telephonic communication service is working at a carrier frequency of $10 \ GHz$. Only $10\%$ of it is utilized for transmission. How many telephonic channels can be transmitted simultaneously if each channel requires a bandwidth of $5 \ kHz$?

$A$ transmitting antenna has a height of $320\, m$ and that of the receiving antenna is $2000\, m$. The maximum distance between them for satisfactory communication in line-of-sight mode is $d$. The value of $d$ is $\ldots \ldots \, km$.