يختار النظارة الطبية والطلاء المضاد للانعكاس بسبب إجهاد العين.

Because Arabic often leaves the subject pronoun unstated when the verb already shows it.

يختار is 3rd person masculine singular, so it already means he chooses / he is choosing.
If you added هو, it would usually be for emphasis or contrast:

هو يختار النظارة الطبية... = He is choosing the prescription glasses...

So the sentence is perfectly normal without an explicit subject.

It can mean either one, depending on context.

The Arabic imperfect verb often covers:

• he chooses
• he is choosing
• sometimes even he will choose if the context suggests future time

So by itself, يختار is not as tightly limited as the English present simple or present progressive.

In Arabic, a pair of glasses can be treated as one object, so the singular نظارة is very common.

So:

• نظارة = glasses / a pair of glasses
• نظارة طبية = prescription glasses
• نظارات = glasses (plural), also possible in other contexts

This is one of those places where Arabic and English package the idea differently.

Because الطبية is an adjective modifying النظارة, and Arabic adjectives must agree with the noun they describe.

Here, النظارة is:

• feminine
• singular
• definite (it has ال)

So the adjective must match:

• الطبية = feminine, singular, definite

This is a very common Arabic pattern:

• السيارة السريعة
• اللغة العربية
• النظارة الطبية

This phrase is built in layers:

• الطلاء = the coating
• المضاد = anti-, opposing, counteracting
• للانعكاس = to reflection / against reflection

So literally it is something like:

the coating that counteracts reflection

In more natural English, that becomes the anti-reflective coating.

Also notice that المضاد agrees with الطلاء:

• الطلاء is masculine singular definite
• so the adjective is المضاد

Because مضاد commonly takes لـ to show what something is opposed to or works against.

So:

• مضاد للبكتيريا = antibacterial
• مضاد للالتهاب = anti-inflammatory
• مضاد للانعكاس = anti-reflective

This is a very useful vocabulary pattern in Arabic.

Because بسبب is used before a noun phrase, while لأن is usually followed by a full clause.

Here we have:

• بسبب إجهاد العين = because of eye strain

That is a noun phrase, so بسبب fits naturally.

Compare:

• بسبب إجهاد العين = because of eye strain
• لأن عينيه متعبتان = because his eyes are tired

So both can express cause, but they are used with different structures.

Yes. إجهاد العين is an iḍāfa construction.

It literally means:

• إجهاد = strain / fatigue
• العين = the eye

So together:

• إجهاد العين = eye strain
• literally: strain of the eye

This is a very common Arabic way to express noun relationships.

Arabic often uses the singular body-part noun in a general or category sense.

So إجهاد العين is the standard way to say eye strain, even though humans normally have two eyes.

If you specifically wanted to emphasize both eyes, you could say:

• إجهاد العينين

But إجهاد العين is completely natural and idiomatic.

Here و simply joins two direct objects of the verb يختار:

• النظارة الطبية
• الطلاء المضاد للانعكاس

So the meaning is that he chooses both:

1. the prescription glasses
2. the anti-reflective coating

That is true: Arabic often uses the definite article more broadly than English.

In a sentence like this, the definite forms can sound natural for:

• specific items already understood in context
• standard product categories
• options being discussed

So Arabic may say:

• النظارة الطبية
• الطلاء المضاد للانعكاس

where English might simply say:

• prescription glasses
• anti-reflective coating

without always using the.

A fully vocalized version is:

يَختارُ النَّظَّارَةَ الطِّبِّيَّةَ وَالطِّلاءَ المُضادَّ لِلانعِكاسِ بِسَبَبِ إِجهادِ العَينِ.

A few useful notes:

• يختارُ ends in ـُ because it is a present-tense verb in the indicative
• النظارةَ and الطلاءَ are direct objects, so they are accusative
• المضادَّ matches الطلاءَ, so it is also accusative
• للانعكاسِ is after a preposition, so it is genitive
• بسببِ إجهادِ العينِ contains genitive relationships as well

In normal everyday writing, these case endings are usually not written.