Here's the
official explanation provided by the GMAC for this question:
In this sentence the word
both introduces the first of two conjoined phrases describing ways that nomads were able to use horses. These phrases must be presented in parallel form, in this case starting with the preposition
as . Idiomatically, when
both introduces the first of two conjoined phrases,
and must introduce the second.
Option A: Since
both introduces the first of two conjoined phrases, it is unidiomatic for
but also to introduce the second.
Option B: Since
both introduces the first of two conjoined phrases, it is unidiomatic for
or to introduce the second.
Option C: Correct.
B oth introduces
as a means of transport, the first of two conjoined phrases.
And introduces the second of these phrases,
as an effective military tool. The two phrases are presented in parallel form, both starting with
as.
Option D: When
both introduces the first of two conjoined phrases, it is unidiomatic for
or to introduce the second. Furthermore,
as a means of transport is not parallel with
an effective military tool; the former phrase starts with the preposition
as, while the latter starts with the article
an and completes the conjunction illogically with a noun phrase rather than an adverbial phrase.
Option E: As a means of transport is not parallel with
also an effective military tool; the former phrase starts with the preposition
as, while the latter starts with the adverb
also.
The correct answer is C.
Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.