The OA A preserves parallelism.

In contrast, E does not preserve it:
Either one has insufficient skills or that one underperformed due to insufficient preparation or pressure.

The "that" is missing in the first part of the solution.
Hope this helps.

Melon,
The correct usage is be 'EITHER X or Y' and X and Y should be parallel.

A) either that one has insufficient skills or that one underperformed due to insufficient preparation or pressure. - Correct!

B) that either one has insufficient skills or that <EXTRA>one underperformed due to insufficient preparation or pressure. - Incorrect answer choice.

C) either that one has insufficient skills or that one underperformed due to insufficient preparation or one was under pressure. The list in red is not parallel. It should rather be Insufficient preparation Or Immense pressure. - Incorrect answer choice.

D) that either one has insufficient skills or that - <EXTRA>one underperformed due to insufficient preparation or not handling pressure well. - Incorrect answer choice.

E) either one has insufficient skills or that - <EXTRA> one underperformed due to insufficient preparation or pressure. - Incorrect answer choice.

Thanks for the post but can you please post in a more legible manner(With the underlined section and spaces) for the benefit of yourself(most experts tend to ignore if the question is posted this way)

[I have edited your post to make it more legible.]

melon wrote:If one scored poorly on the GMAT, it could mean either that one has insufficient skills or that one underperformed due to insufficient preparation or pressure.

MASTER GMAT:
Click individual answer choices for specific explanations.
A) either that one has insufficient skills or that one underperformed due to insufficient preparation or pressure.

B) that either one has insufficient skills or that one underperformed due to insufficient preparation or pressure.

C) either that one has insufficient skills or that one underperformed due to insufficient preparation or one was under pressure.

D) that either one has insufficient skills or that one underperformed due to insufficient preparation or not handling pressure well.

E) either one has insufficient skills or that one underperformed due to insufficient preparation or pressure.

OA A please explain me why e is wrong. is it just parallelism

Whenever we look for parallelism, it should looked in the following way.
Either X or Y

And clearly X and Y should be parallel.
In A, there is "THAT" and "THAT" parallelism which is not present in E.

If you will see that C does have THAT and THAT parallelism but the construction,due to insufficient preparation or one was under pressure is not correct.
For parallelism,
There can be parallelism in prepositional phrase, adjectives, verbs , nouns and that clauses.
Hope this helps.

Thanks