If your trouser fly is unfastened what should you do?
Options:
- do up
- tie up
- zip up
- pick up
- button up Let's think step by step:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
You can zip up your trouser. If your trouser fly is unfastened you should zip up it. So, the answer is zip up.