(b) but he also
The correct answer is (b) but he also. The sentence uses the correlative conjunction pair "not only … but also," which is the standard structure for adding emphasis.
"Not only … but also" is a fixed correlative conjunction pair used to add a second (often more impressive) point. The other options break grammatical or idiomatic rules: "as well as" doesn't pair with "not only," "and he also" is grammatically weak here, and "just as he" changes the meaning entirely. Examiners expect students to recognise correlative conjunctions in fill-in-the-blank grammar questions.