If one scored poorly on the GMAT, it could mean either that one has in

Official Explanation

THE PROMPT
• Parallelism

1) Either X Or Y

In the idiomatic construction Either X or Y, the X and Y elements must be parallel.
That is, the X and Y elements must be similar parts of speech and must play the same grammatical or logical role in the sentence.

GMAC likes to test whether you understand where to place words such as
-- prepositions
Correct: for either X or Y ↔ either for X or for Y

-- subordinating conjunctions
Correct: . . . demonstrates that either X or Y ↔ . . . demonstrates either that X or that Y

-- the first part of a "split" verb
Correct: He has either been fined or jailed. ↔ He has been either fined or jailed.

In my examples, both versions are correct.

Look at what immediately follows the X and Y terms.
Whatever follows should be parallel.

That said, I encourage you to try to see the pattern rather than follow a mechanical rule.
See whether you can create an example similar those I wrote above, in which you produce two versions of Either X or Y, using the word because (a subordinating conjunction) or the word from (a preposition).


2) A or B
This construction, too, requires that elements be parallel.

THE OPTIONS
• no error
There are two lists in this question.
1. either X or Y
2. A or B

In this answer, X and Y, and A and B, are correctly of the same part of speech in both lists:
1. either that... or that...
2. noun (preparation) . . . noun (pressure)

KEEP

• not parallel
The same part of speech that follows either must also follow or.
Either is followed by the pronoun one.
Or, however, is followed by the conjunction that.
Eliminate B

• Not parallel

There are two lists in this question [that involve two idioms, Either X or Y, and A or B].
In this answer choice, the first list, constructed with Either X or Y, is correct: either that... or that...

The second list, though, is incorrect.
It is in the format of A or B.
A is a noun (insufficient preparation) while B is a clause (one was under pressure).
(A clause contains a subject and a verb and is much different from a noun or noun phrase.)
Not parallel.
Eliminate C

• not parallel
There are two lists in this question.
1. Either A or B
2. A or B

The first list is incorrect.
The second list is acceptable but not preferred (noun-gerund is not preferred to noun-noun).

In the first list, either is followed by the pronoun one while or is followed by the conjunction that.

In the second list, the first item is a noun (insufficient preparation) while the second item is a [gerund phrase (not handling pressure very well)], which would be better if it were written with a dedicated noun such as "pressure."
Eliminate D

• not parallel
The same part of speech that follows either must also follow or.
Either is followed by the pronoun one.
Or, though, is followed by the conjunction that.
Eliminate E.

The answer is A.

• NOTES

• The pronoun "one" - some thoughts

→ In English, if we want to talk about "a person in general" (or to connote "people in general"), we use the pronoun one.
In U.S. English, this usage is fairly rare; in the States, one sounds stuffily formal and a bit weird.

Nonetheless, you should know that:
(1) the verb conjugation of "one" is the third person singular
→ one remembers, he remembers, she remembers
→ one learns, she learns, he learns

(2) GMAC may test you on one.
→ If so, the incorrect answers will typically contain a combination of he or she or you, on the one hand, and one on the other hand.
Do not mix the pronouns.
Wrong: If one applies for asylum in the U.S., he or she must demonstrate persecution based on one of five protected grounds, namely, race, religion, nationality, membership in a particular social group or political opinion.

Correct: If one applies for asylum in the U.S., one must demonstrate persecution based on one of five protected grounds, namely, race, religion, nationality, membership in a particular social group or political opinion.

Correct: If a person applies for asylum in the U.S., he or she must demonstrate persecution based on one of five protected grounds, namely, race, religion, nationality, membership in a particular social group or political opinion.

Many native speakers use you to talk about "a person/people in general," this way:
→ Informal: For inspiration about perseverance against all odds, you should learn about Nelson Mandela.
Although native U.S. speakers frequently use this construction in informal speech, avoid the construction in formal writing.
(Professional writers often get around the issue entirely and use neither you nor one.)

In formal English, we use one rather than you, this way:
→ Formal: For inspiration about perseverance against all odds, one should learn about Nelson Mandela.

Although this question does not test pronoun usage, its syntax and tone invite the use of one, a usage that GMAC tests on occasion—so I thought I'd give you a heads up.

COMMENTS

PriyamRathor , welcome to SC Butler.

For the sake of the community on this forum, I am glad to see a mix of newcomers and veterans.
I encourage aspirants to take up my standing invitation to post.
If you can teach it, you understand it.
Don't worry if "your" solution has already been posted.
Write what you think in your own voice, in part to develop that voice and in part to help someone else who thinks in ways similar to you—that someone will read the thread and something about your post will hit home.

Happy Ramadan to those who observe.

Keep up the hard work, everyone.
You are getting there.