Explanation

Just as… so too… suggests that the mainstream novels show a gift, just as the sciencefiction novels do. The first word should mean something like “show,” or “display.”

Among these answers, only attest has that meaning. Incorrect choices belie and gainsay both indicate an opposite direction, which is incorrect, since the just as… so too… structure indicates same direction. The second word should describe the world of mainstream, rather than science-fiction, novels. You want a word that means “not fantastic,” “ordinary,” “everyday.” Quotidian has just that meaning.

Attest to, quotidian.