At ground level, ozone is a harmful pollutant, but in the stratosphere

B, C and E require the additional verb.e.g.
In my class, Kate is intelligent, but in another class in which/where there are other bright students, she is not noticeable. The example above must have the similar structure to be right. E is wordy. So A remains correct.

Because of the very vastness of grammar, I have always found the elimination method to get correct answer in SC.

I will try to reason it via "Process of Elimination" :

(A) in the stratosphere Parallel to "at the ground level" I donot find any error. So lets leave it now
(B) in the stratosphere, in which - which is unnecessary. Eliminated
(C) it is in the stratosphere in which Same as B
(D) in the stratosphere where - Where is unnecessary. Eliminated
(E) it is in the stratosphere and - No need of conjunction. Eliminated.

Now we can see, we are left with only option A. And others have clear errors.
HENCE A.

(A) in the stratosphere - Correct
(B) in the stratosphere, in which - Wrong: 1) Parallelism issue. 2) Structure issue
(C) it is in the stratosphere in which - Wrong: Parallelism issue
(D) in the stratosphere where - Wrong: 1) Parallelism issue 2) Structure issue
(E) it is in the stratosphere and - Wrong: 1) Parallelism issue 2) Meaning issue

why in answer A we did not have a comma after but !?
if it must be parallel ,why we did not separate "in the stratosphere" by comma

It is not essential to put commas around in the stratosphere.

More generally, keep in mind that the GMAT tests commas in a very limited number of ways (such as comma splices), and this is not one of them.

At ground level, ozone is a harmful pollutant, but in the stratosphere it shields the Earth from the most biologically harmful radiation emitted by the Sun, radiation in the ultraviolet band of the spectrum.

May I know why a comma is required after "but"?

If you put the conjunction 'but' in parenthesis, then it becomes dispensable to the meaning. Without the conjunction, the whole sentence might become a fused sentence.

I doubt whether a comma is normally used after a co-ordinate conjunction

Because 'but' is not a modifier. it is a conjunction. On the other hand, at ground level is a prepositional modifier. Modifiers are usually separated from what they modify by a comma.
At the same time, could you see that the phrase 'in the stratosphere' is not separated by a comma from what it follows. That is because that phrase is indispensable to the meaning. You may that without the prepositional modifier the second arm has no sensible meaning.

Let’s keep Option A. It seems worth holding on to.

Eliminate Option B for repeating ‘in’ again – it is redundant.

Eliminate Option C for repeating ‘it’ and ‘in’ – super awkward.

Eliminate Option D for ‘where’ – illogical when you substitute back into the sentence.

Eliminate Option E for breaking the parallelism that is intended in the original sentence - 'it ... and'.

Option A is the best choice.

Hope this helps!

4. At ground level, ozone is a harmful pollutant, but in the stratosphere it shields the Earth from the most biologically harmful radiation emitted by the Sun, radiation in the ultraviolet band of the spectrum.
(A) in the stratosphere
(B) in the stratosphere, in which
(C) it is in the stratosphere in which
(D) in the stratosphere where
(E) it is in the stratosphere and

Often and odd, many students miss such simple questions, because they are not aware of basic sentence patterns and their flaws. You see, there is a ‘but’ preceded by a comma and it indicates that what follows ‘but’ must be an IC to match the IC before the co-ordinate conjunction. On the sense, choice B,C, and E are fragmented choices due to the untoward intrusion of the relative pronoun’ Which’ and the relative adverb ‘Where’. This is what you can get as lucky as 75% or three of the four choices wrong.
E does not convey any sensible meaning. Therefore, A is the correct choice.

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