Answer:
Owned up
Step-by-step explanation:
As a result of the topic being idioms/phrases, the best word that completes the sentence is the idiom "owned up".
Another word that would have sufficed would be "admitted" but unfortunately, it isn't an idiom, so the idiom "owned up" is the correct answer.
The idiom "own up" means admitting to something.
"When his father was ready with cane,he OWNED UP to having stolen the ring."