The correct answer is C
Explanation
The error in this sentence occurs at (C), where there is an improper idiom. In order to appropriately complete the correlative construction, the word “also” must be inserted after the conjunction “but.” This results in the correlative construction “not only … but also.” In addition, what follows “not only” must be grammatically parallel to what follows “but also,” so the word “but” should be changed to “but also because.”