Answer:
The best option for the idiom is A) at risk.
Step-by-step explanation:
When we talk of someone being or treading on dangerous ground, we mean that person is at risk. The risk will depend on the situation. They may be at risk of offending someone and making them angry, or of obtaining a bad result for a reckless action, for instance.
- You're treading dangerous ground lying to your father this way.
- She's on dangerous ground after missing the meeting this morning.