Explain forced and resonance oscillations

Two pendulums $C$ and $D$ are suspended from a wire as shown in the given figure. Pendulum $C$ is made to oscillate by displacing it from its mean position. It is seen that $D$ also starts oscillating. Name the type of oscillation, $D$ will execute.

A pendulum with the time period of $1 \mathrm{s}$ is losing energy due to damping. At a certain time, its energy is $45 \mathrm{J}$. If after completing $15$ oscillations its energy has become $15 \mathrm{J}$, then its damping constant (in ${\mathrm{s}}^{-1}$) will be

The amplitude of a simple pendulum, oscillating in air with a small spherical bob, decreases from 10 cm to 8 cm in 40 seconds. Assuming that Stokes law is valid, and ratio of the coefficient of viscosity of air to that of carbon dioxide is 1.3, the time in which amplitude of this pendulum will reduce from 10 cm to 5 cm in carbondioxide will be close to (ln 5 = 1.601, ln 2 = 0.693).

A vibrating tuning fork is placed over the mouth of the burette filled with water. The tap is opened and the water level gradually falls, it is found that the sound becomes very loud for some particular length of water column. Why does the sound become very loud?

A vibrating tuning fork is placed over the mouth of the burette filled with water. The tap is opened and the water level gradually falls, it is found that the sound becomes very loud for some particular length of water column. State the name of phenomenon taking place when this happens.

A vibrating tuning fork is placed over the mouth of the burette filled with water. The tap is opened and the water level gradually falls, it is found that the sound becomes very loud for some particular length of water column. What is the name of phenomenon when sound is produced for other length of air column but it is not very loud?