1. „To know your onions“ stands for:
Answer B (being knowledgable about a subject) is correct. Example: „Our accountant is a true expert. When she was talking to the tax authorities she certainly showed she knew her onions.“
2. To be „as cool as a cucumber“ means:
Answer B (to stay calm in a difficult moment). Example: „Jane is an amazing woman. During the accident she remained as cool as a cucumber.“
3. Who is „the big cheese“?
Answer A (the boss or the most important person) is correct. Example: „We should better talk to the big cheese first. It’s an important decision that may affect our business for years.“