Sentence Correction (SC) | Re: The largest of all the planets, not only is Jupiter three

In the official explanation, it is mentioned that idiom form "not only X but Y" is used and not idiom form "not only X but also Y" is used. But how to identify which form of "not only" idiom is used in the given sentence?
Because in "not only X but also Y" form of idiom "X" and "Y" have to be parallel. Thinking that I chose E
But here