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**Hint:**Kundt’s tube: It is made up of a transparent horizontal pipe-like instrument that contains a small amount of a fine powder such as cork dust, talc, or lycopodium.

At one end of the tube, there is a source of sound at a single frequency (a pure tone).

A metal rod resonator that caused vibration or 'ring' by rubbing the rods, but nowadays in modern demonstrations usually use a loudspeaker attached to a signal generator.

**Formula used:**

${\text{f = }}\dfrac{{\text{c}}}{\lambda }$, here f = frequency of the sound wave,

c= speed of the light source,

$\lambda $= wavelength of the sound wave

**Complete step-by-step solution:**

In 1866, German physicist August Kundt invented Kundt's tube as an experimental acoustical instrument.

It is used to measure the speed of sound in a gas or a solid rod.

The experiment is still used due to its ability to demonstrate longitudinal waves in a gas (which can often be difficult to visualize).

Nowadays, it is used for demonstrating standing waves and acoustical forces.

It is given that ${\text{d = 20cm}}$,

Here d= separation of the heaps

Therefore, we can write that, ${\text{d = }}\dfrac{\lambda }{2} = 20{\text{cm}}$

From the above equation, we get the value as $\lambda = 40{\text{cm}}$

Then, we can find the value of frequency, ${\text{f = }}\dfrac{{\text{c}}}{\lambda }$

Putting the values we get,

$ = \frac{{330}}{{0.4}}$

Let us divide the term we get,

$ = 825{\text{Hz}}$

**Hence the correct option is (B)**

**Note:**- Lycopodium powder is yellow-tan dust-like powder.

-Lycopodium powder consists of dry spores of club moss plants, ferns.

-When lycopodium powder is mixed with air, the spores formed are highly flammable.

-Kundt’s tube is used to find the wavelength of sound with the cork-powder method.

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