Question

Σ

なぜWhat とWhateverはダメなのですか？

That) she believes you is certain.
[] That/What/Whatever/Which
[#4] That

Answer 1

おゆ

whatとwhateverは後ろに不完全文を取ります。
今回は後の文がshe believes you、つまりSVOで完全文なのでwhat,whateverは取れません。