Choose the correct meaning of the following idiom:

Zero hour

At a scheduled time

The correct meaning of the idiom "zero hour" is at a scheduled time. It is often used to refer to the time when a military operation or other important event is scheduled to begin. The idiom is thought to have originated from the fact that military operations were often scheduled to begin at 00:00 hours, or "zero hour."

The other options are incorrect. "At no scheduled time" means that there is no specific time for something to happen. "At any time" means that something can happen at any time, without any prior notice. "At 12" means that something will happen at 12:00 hours, or noon.