Michael Jackson is 7,7 and he had many number one hits in the 1990s and not just one.
The only artist fitting 7,8 is Outhere Brothers, (with one number one in the 1990s) but they are performers rather than a performer
The answer that would fit the being one of five is Michael Jackson. (The Jackson Five), so there is a mistake in the numbering. (Strictly speaking they should have said had number one hits in the 1990s as well!)