The polymer used for optical lenses is :

We begin by recalling that an optical lens must be perfectly transparent to visible light, mechanically strong, scratch-resistant, and, in many situations, lighter than glass. So the polymer selected for making such lenses has to possess excellent optical clarity (high light transmission and low haze) and adequate rigidity.

Now we examine each option one by one and discuss whether its properties match these stringent requirements.

First, polypropylene (PP). This polymer is written as $$\,(CH_2CH(CH_3))_n\,.$$ Although polypropylene is light in weight and chemically resistant, it is only semi-crystalline and displays a slight opaqueness called “translucency.” Because of this haze, PP cannot be employed where perfectly clear vision is mandatory, so it is not suitable for precision optical lenses.

Next, polyvinyl chloride (PVC). The repeating unit of PVC is $$\,(CH_2CHCl)_n\,.$$ The presence of the chlorine atom not only adds density but also imparts a slight yellowish tint. PVC is therefore never colorless enough for high-grade lenses. Moreover, the presence of plasticisers (needed to make PVC flexible) further degrades optical purity. Hence PVC is ruled out for optical applications.

Then we look at polythene, more precisely high-density polyethylene (HDPE) or low-density polyethylene (LDPE), whose repeating unit is $$\,(CH_2CH_2)_n\,.$$ Polyethylene is a highly semicrystalline polymer: its crystalline regions scatter light strongly, producing a milky appearance. Because of that light scattering, polyethylene sheets appear whitish, not water-clear. Therefore polyethylene is also unsuitable for precision lenses.

Finally, we consider polymethyl methacrylate, abbreviated PMMA, whose structural repeat unit can be written as

$$\big(CH_2C(CH_3)(COOCH_3)\big)_n.$$

PMMA is commonly marketed under trade names such as “Plexiglas,” “Perspex,” and “Lucite.” A key property of PMMA is its extremely high optical transparency—its light transmission in the visible region exceeds $$92\%,$$ which even surpasses that of many kinds of glass. In addition, PMMA has a high refractive index, satisfactory scratch resistance, and low density, making lenses lighter to wear. Because of these combined attributes, PMMA is routinely molded or cast into ophthalmic lenses, camera lenses, automobile taillight covers, and other optical parts.

Comparing all four candidates, only polymethyl methacrylate fulfills every essential criterion for an optical lens material.

Hence, the correct answer is Option D.