While making a kaleidoscope, a transparent plastic sheet is pasted under the cardboard disc in order to:
A. Give it an attractive look
B. Make the disc durable
C. Hold some air inside
D. Make patterns more colourful

Hint: Kaleidoscope is an instrument which works on the principle of multiple reflections. It is an instrument which has two or more reflecting surfaces. These surfaces are tilted towards each other at a certain angle. The structures are observed due to the mirrors, pebbles and beads in it. Without beads and pebbles, no structures can be observed.

Complete step-by-step answer:
Kaleidoscope is derived from Ancient words known as ‘kalos’ which means beauty, ‘eidos’ which means something that is seen and ‘skopeo’ means to look. Kaleidoscope is a cylindrical tube with mirrors containing loosely coloured objects such as beads, pebbles and bits of glass. The phenomenon of repeated reflections is used in it. These mirrors are kept at a certain angle to each other to produce a large number of reflections.
While making a kaleidoscope, a transparent plastic sheet is pasted under the cardboard disc in order to make the disc durable.
So, the correct answer is option D i.e. make the disc durable.

So, the correct answer is “Option D”.

Note: If we want to increase the number of designs formed in the kaleidoscope then increase the number of mirrors in it. This will increase the number of surfaces from which reflection takes place. Same pattern cannot be seen in a kaleidoscope again. It gives a different pattern every time. This is due to multiple reflections of light which is because of the tilted formation of strips. If we keep two mirrors at an angle $\theta$ then the number of reflections of a single object is given by, $(\dfrac {360}{\theta}-1)$.