Gasoline marketing is undergoing major changes as stations

[Capthan]

Gasoline marketing is undergoing major changes as stations often not only add convenience stores but also combine with major fast-food chainsto built complexes where customers can shop and eat as buy gasoline.

-not only add convenience stores but also combine with major fast-food chains to build
-not only add convenience stores but also they combine with major fast-food chains to build
-are not only adding convenience stores but also in combination with major fast-food chains they build
-add not only convenience stores, but also combining with major fast-food chains to build
-are not only adding convenience stores, but also in combining with major fast-food chains they build

Please explain your answer. Thanks

[H]

A is correct.
B is not - "they" should be omitted or a comma is needed preceding "but also".
C, D, and E are incorrect because the construction of "not only...but also..." requires a strict parallelism, and they don't have it.

[RonPurewal]

it appears that you've made a couple of typographical errors in the original sentence.

to resolve this problem, it's sufficient to consider the parallel structure required by the construction 'not only ... but also', which appears in all five answer choices.
only choice (a) contains elements that are actually parallel: not only add convenience stores, but also combine with major fast-food chains...
in choice (b), the inclusion of the word 'they' in the second part of the parallel construction is fatal.
in choice (c), you probably couldn't make the two parts less parallel if you tried!
in choice (d), the first part of the parallel construction is just a noun (convenience stores), but the second part is a big fat clause thingy.
in choice (e), the lack of parallelism is clear.