Yes, you are right. Here "using" is wrongly used as a present participle modifier. Option D is the OA. Here "using" is not a present participle modifier, but a gerund parallel to "offering". These two gerunds are joined by the conjunction "but". The structure in D is as follows:

.. by offering but using..

I see what you mean.. What you're saying makes total sense. I can see that using as a VERB-ING modifier for "Many airline carriers are attempting to increase profitability" makes sense.
So we reject E because of the faults:

1. an equal number and "were" (plural) do not go along together
2. We are using "were" in the answer choice, but we do not have a form of "be" in the part before

Is that correct ? Thanks for the insights, I appreciate them

I feel this is saying that

they offered last year, using larger planes that fly more efficiently

This is suggesting that last year, they used larger planes. The sentence wants to say that this year, they used larger planes.

Is my understanding correct?

Many airline carriers are attempting to increase profitability while keeping overhead low by offering, in terms of flights,an equal amount as last year, doing so by using larger planes that fly more efficiently.

A. an equal amount as last year,doing so by
B. the same number offered last year
C. an equal amount offered last year and
D. the same number as last year but
E. an equal number as were offered last year,

Amount:when the entity is not countable or abstract noun.
Number:when the entity is countable
(eg money not countable:amount,Dollars is countable:number)

this question test the use of "the same ... as".

"the same ... as" is different from other comparisons. the pattern with " the same ...as" alway give us the feeling that the sentence is wrong.
we do not say,
I have the same pen as your pen.
but we say
I have the same pen as you. (this dose not mean "pen" is compared with "you". in this correct pattern, there is no preposition after "as" and we get the feel that the sentence is wrong. this point is the reason for which I say this pattern is different from other comparisons.

I am stronger in summer than in winter. I am better at gmat than he is
Here, we have "in" before "winter" and "is" in second clause and so, we realize quickly that the sentence is correct because comparison elements have prepositions or helping verbs which make those elements clear.

so, definitely "the same as last year", " the same as you" are all correct. and remember, in "the same ...as" pattern, we normally do not have preposition and helping verb, and, so, the absence is the reason for which you feel the sentence is wrong.

in short, the absence of preposition or helping verb is specialty of the pattern "the same ...as".

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Serega
I find a slight confusion in your query. Your query is about choice D and there is nothing as an equal amount as last year in D. It only says the same number as last year. Are you referring to C? If you say that choice D stands on ellipsis, can you please state what are the appropriate words that are elided. This may help us to understand your query more clearly.
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I'm not 100% sure that I'm interpreting your question correctly, but here is the sentence using choice (D):

"Many airline carriers are attempting to increase profitability while keeping overhead low by offering, in terms of flights, the same number as last year but using larger planes that fly more efficiently."

The "as" construction compares the number of flights, not the amount of keeping overhead low (i.e. "... keeping overhead low by offering the same number [of flights] as last year). In other words, the number of flights offered in the present year is equal to the number of flights offered last year.

I hope this helps!
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Here is my analysis,
Firstly, we are talking about the number of flights, which is countable and hence options A and C are out. (Both of them use "amount")
Option B: the same number offered last year using larger planes that can..... There is a clear shift in meaning here. "Using larger planes..." now modifies "Last year". However, the original sentence wants to communicate that it is the current year when the airlines are using larger planes.
Option E: an equal number as were offered last year : This choice has a clear pronoun number disagreement. It uses "were" to compare itself with a singular noun "an equal number". Eliminate option E
Option D: Remember at this point, we have eliminated all the remaining choices successfully. In real GMAT, at this point it's better to select it and move on than to ponder about why should it be a correct answer. Assuming that now you are doing a thorough review of your mock, you need to know precisely why is this the correct option!

My opinion: At first glance, the choice might seem to have a comparison error. However, when I re-read the complete answer, I could justify the choice as correct. The sentence uses "by offering, in terms of flights," to make it clear that we are comparing the number of flight and nothing else. Hence, in strictly meaning perspective, this choice is sufficiently clear.

Hi Skywalker18!

Here, the comparison is between the number of flights this year and the number of flights last year. Some of those words are omitted for clarity, but there is no problem with the construction See victory47's post earlier for an explanation of the comparison here!

-Carolyn
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Subtle and interesting split.

Number ----- number of flights.
Amount-------its not the price of ticket by which they want to increase the profitability.

If you check from B and D you can get the solution by meaning.
D is the winner!
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