Quel(s), quelle(s), qu'elle(s) : how to spell well; in one word, in two words?

Quel(s), quelle(s), qu'elle(s) : how to spell well; in one word, in two words? What is the word "that"?
Etymology of quel, etymology of qu'elle, etymology of que. Spelling of quel, spelling of qu'elle. Quel, quels, quelle, quelles, qu'elle, qu'elles.
Spelling. Vocabulary. Learn French. Write well.