Which type of medium is used for optical communication?

Match the following:

List-$I$ (Signal)	List-$II$ (Bandwidth)
$i$. Speech Signal	$a$. $4.2 \text{ MHz}$
$ii$. Music Signal	$b$. $6 \text{ MHz}$
$iii$. Video Signal	$c$. $20 \text{ kHz}$
$iv$. $T$.$V$. Signal	$d$. $2.8 \text{ kHz}$

For the output of an $AM$ detector circuit to follow the envelope of the carrier wave,which condition must be satisfied? ($T_c$ is the time period of the carrier wave.)

$A$ signal of frequency $10 kHz$ and peak voltage $10 V$ is used to amplitude modulate a carrier of frequency $1 MHz$ and peak voltage $20 V$. The side-band frequencies in $kHz$ are

Basically,the product modulator is

What is the frequency of an $FM$ transmitter without a signal input?