Typically, the words "but also" are used in a construction following "not only." At the very least, you should expect some kind of signal that a contrast is coming before "but," such as "although." (A) can be eliminated. (B) and (E) are incorrect for the same reason.

The first part of (C) is awkwardly written, but the bigger problem is the verb choice at the end of the underlined portion: "had begun." There is only one point in the past in this sentence: 1997. "Had begun" is past perfect, implying multiple points in the past. Thus, (C) is incorrect. (D), with its use of "began" instead, is the correct choice.