Choose the correct word to complete the detective's dramatic courtroom revelation.

"Ladies and gentlemen of the jury, ___ the defendant hid inside that oversized tuba is exactly the missing diamond we've been searching for!"

The correct answer is what.

Here, what functions as a nominal relative pronoun (meaning "the thing that"). It acts simultaneously as the object of "hid" and as the head of the noun clause that serves as the subject of the main sentence. Using that would create a declarative fact ("The fact that he hid... is the diamond"), which makes no logical sense in this context!