Your shoelace isn't tied. () tie it, or you'll fall down.

• A. You'd better
• B. You should
• C. You must
• D. You could

Answer: (A)

This tests how you express urgent, warning-like advice about avoiding a bad result. You'd better tie it because this phrase signals that action should be taken now to prevent a negative outcome, with the consequence made explicit in the clause that follows (“or you’ll fall down”). It’s a natural, common way to push someone to act firmly, stronger than simply saying you should, but not as rigid as a must. The other options change the tone: should is milder advice, must sounds like a strict rule, and could would just suggest a possibility without urging action.