A- The transition but rather should not introduce a clause, but it is starts a new clause.

B- Government-owned companies exist is not parallel to the other two items on the list (the amount… and the extent…)

C- A which phrase set off by commas grammatically indicates non-essential information; since the information after which is logically necessary for this sentence to make sense, this answer choice is wrong.

D- Correct: This answer choice uses but as part of an idiomatic expression not x, but rather y. This is an exception to the typical rules governing connecting clauses, so it is correct.

E- The phrase but also can only be used with the idiomatic expression not only. Government-owned companies exist is not parallel to the other two items on the list (the amount… and the extent…)