Which one of the following materials cannot be used to make a lens?

The correct answer is Option (4) → Clay

A lens works by letting light pass through it and bending (refracting) it, so it must be made of a transparent material with curved surfaces. Glass and clear plastic are the usual materials, and even a drop of water acts as a small lens. Clay is opaque: it does not let light through, so it cannot be used to make a lens.