Option (D) Clay is correct. According to the context, 'A transparent material bound by two surfaces, of which one or both surfaces are spherical, forms a lens.' This definition clearly states that a lens must be made of a transparent material. Water, glass, and plastic are all transparent materials that allow light to pass through and can be shaped into lenses. Water has a refractive index of 1.33 as shown in Table 9.3, and glass is extensively discussed as a lens material in the context. Clay, however, is an opaque material that does not allow light to pass through it. Since it is not transparent, clay cannot refract light and therefore cannot be used to make a functional lens.