A few issues with D:

*"Unlike" is a problem because it makes us compare ramjets to ramjets! Notice that a scramjet is a kind of ramjet. It would be like saying "Unlike dogs, chihuahuas, which are small dogs, do not need much food." Chihuahuas may be unlike *most* dogs, but we can't say they are unlike dogs in general!
*"The ramjets" is wrong. We aren't talking about a certain group of ramjets, so "the" is inappropriate. It's like saying "I like the apples." If you aren't talking about a specific group of apples, it makes no sense.
*The modifier starting with "generally unable" is wrong. It is taking the form of an adverbial modifier, so we'd actually expect it to modify the following clause: "a scramjet . . . can attain high speeds." So we're saying "Unable to achieve high speeds, a scramjet can attain high speeds." Of course this is impossible. Now if what I'm saying doesn't seem accurate to you, it's perhaps because we have *two* initial modifiers in a row in this choice, making it hard to tell what the sentence is really trying to say.

anje29, the issues you mentioned are actually not flaws in D. (It has enough!) It's fine to compare a plural noun with a singular noun, and we don't need to have to words "can" and "cannot" to have parallelism. For instance, it would be fine to say "Sloths do not run very fast, but a cornered sloth can strike quickly."
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Both the conjunctions - 'whereas' & 'although' has different usage.

Although - it is used to connect 2 different clauses which are related to one another. Some thing like despite the fact or but
Although she knew that it was dangerous, she walked home by herself.
Although I begged him not to, he decided to go.

Whereas - is used to compare to things totally unique by themselves. Like comaprison of apples to oranges on a common attribute

He must be about sixty, whereas his wife looks about thirty.
You eat a massive plate of food for lunch, whereas I have just a sandwich.

Moreover in 'E' the phrase 'does not have the capability' is wordy and it is out of synch with can attain

I'll pick B. The highlighted section of the sentence can be simplified to read as "Unlike X, Y", which is what option B is all about. Though D reflects a similar construction, the organization is whacky and not as crisp as B

Another B for proper comparision: a ramjet (singular) with a scramjet (singular).

I'd also pick B. D is tempting, but it uses plural "ramjets", while in the second part is singular "scramjet"

B

Adding to what everyone said - it is also || "cannot achieve, can attain"

OA is B. I chose (B) but is anything wrong with (E)?

E is wrong because the correct idiom is "capability to."

why E is wrong?

because it is wordy. that is the only reason for which we eliminate it

(A) Whereas high speeds generally cannot be achieved by ramjets without their initially being assisted by
----1. AWKWARD and WORDY

(B) Whereas a ramjet generally cannot achieve high speeds without the initial assistance of ------ The "Best" option

(C) With ramjets, high speeds generally are not able to be achieved without initially being assisted by
----1. With ramjets is not parallel to "a scramjet"
----2. being assisted by is wordy and awkward

(D) Unlike the ramjets, generally unable to achieve high speeds without the initial assistance of
-----Modifier issue, There should have been a "which" before generally

(E) Although a ramjet generally does not have the capability of achieving high speeds without the initial assistance from
-----Meaning has been changed[/list]
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with regard to option D, can someone clarify this or me:

1. is the use of a plural the only reason why the option is wrong?

2. If we were to use a singular, is the use of unlike ok?

3. Does the use of 'the' before ramjets distort parallelism in any way?

A - 'being' is unnecessary + the comparison is between a ramjet and a scramjet, and NOT high speeds and a ramjet. OUT.
B - No errors. KEEP.
C - 'are not able to be achieved' and 'being assisted by' - Extremely verbose. OUT.
D - This makes it sound like the comparison is between ramjets, and achieving high speeds. 'Unlike' signals a comparison is to follow, but one never did. OUT.
E - Unbelievable verbose! Between this and B, B wins.

B is the answer.
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Hi akshayk, how does 'unlike' make the comparison between ramjets, and achieving high speeds.

isn't ' generally unable to achieve high speeds without the initial assistance of a rocket' an appositive here?

Also, can you please have a look at my previous post and solve my doubt with other doubts on option D?

Unlike The Game 'Triple H', Stone Cold loves beer.
'the' has nothing to do with parallelism
With Like/Unlike, we just need to make sure that a 'noun' is compared to a 'noun'.
'the' and 'a' are both articles.

Hope this helps
If you still have doubts, request for an expert reply. They should be able to guide you better.
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Dear Expert,
Please clarify why B is better than D. I assume there are 2 points :
1. Parallel cannot , can.
2. In D ramjets is plural.

A " being " is wrong is A , "their " is ambiguous and the sentence is written in passive .
B is correct
C there is no contrast as the original sentence wants to convey also use of preposition with is wrong .
D The is meaning error and it is fragment without main verb .
E it long and wordy, there is a change in meaning it is saying that it is not capable to achieve high speeds .

Careful, arvind910619. Every incorrect answer choice has to actually be wrong in some clear way. Choices aren't wrong simply because of length/wordiness, passive voice, or the word "being." Try running back through this thread and seeing if you can identify grammar/meaning errors in all the incorrect choices.
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(A) Whereas high speeds generally cannot be achieved by ramjets without their initially being assisted by
----1. AWKWARD and WORDY

(B) Whereas a ramjet generally cannot achieve high speeds without the initial assistance of ------ The "Best" option

(C) With ramjets, high speeds generally are not able to be achieved without initially being assisted by
----1. With ramjets is not parallel to "a scramjet"
----2. being assisted by is wordy and awkward

(D) Unlike the ramjets, generally unable to achieve high speeds without the initial assistance of
-----Modifier issue, There should have been a "which" before generally

(E) Although a ramjet generally does not have the capability of achieving high speeds without the initial assistance from
-----Meaning has been changed[/list]

I check although and whereas in the dictionary. yes, they are different.

although is used to show a suprise. whereas is used to show a contrast.

although i learn a lot, I fail on gmat
although i learn less, my gmat score is higher this time

although is not fit inhere, so, choice e is wrong.

maybe i will post again on this point. I am not sure about this point