Choose the correct phrase to complete the detective's dramatic reveal.

The fingerprints were wiped clean, but what _____ that the security cameras had been disabled exactly two minutes before the heist.

The correct answer is really puzzled the investigators was.

In this wh-cleft sentence, the entire noun clause ("what really puzzled the investigators") acts as a singular subject. Therefore, it requires the singular verb "was".

Even though the word "investigators" is plural, it is the object of the verb "puzzled" inside the clause, not the subject of the main sentence. We use the past tense "was" to maintain agreement with the past narrative of the crime.