Which is/are the correct usage(s):
- "What is that key for?"
- "What for is that key?"
- "For what is that key?"
First one looks correct. But what about others? Also, it would be better if you can explain.
What is that key for? is correct. It's what people say and write. The other two examples are not constructed as English speakers would say them. In fact, the second one is quite wrong. The third one might once have been considered correct as there is a misguided perception that sentences should not end with a preposition. From The Chicago Manual of Style Online 5.180:
The traditional caveat of yesteryear against ending sentences or clauses with prepositions is an unnecessary and pedantic restriction. And it is wrong. As Winston Churchill is said to have put it sarcastically, “That is the type of arrant pedantry up with which I shall not put.” A sentence that ends in a preposition may sound more natural than a sentence carefully constructed to avoid a final preposition. Compare, for example, This is the case I told you about with This is the case about which I told you. The “rule” prohibiting terminal prepositions was an ill-founded superstition based on a false analogy to Latin grammar. Today many grammarians use the dismissive term "pied-piping" for this phenomenon.
I have not included a link, as CMOS is behind a paywall.