For sky wave propagation of a $10\, MHz$ signal, what should be the minimum electron density in the ionosphere?

$A$ carrier wave $60 \sin(2\pi \times 10^6 t)$ is modulated by a signal $15 \sin(300\pi t)$. The modulation index is: (in $\%$)

If a message signal of frequency $10 kHz$ and peak voltage $12 V$ is used to modulate a carrier wave of frequency $1 MHz$, the modulation index is $0.6$. To make the modulation index $0.75$, the carrier peak voltage should be

For $100\%$ modulation,what should be the value of the minimum amplitude $(E_{min})$ of the modulated wave?

Assertion $(A)$: Television signals are received through sky-wave propagation.
Reason $(R)$: The ionosphere reflects electromagnetic waves of frequencies in the range $(3-30)$ $MHz$.
Choose the correct option.

In a communication system,noise is most likely to affect the signal: