Android indicate that user have to fill a field before validate

I'm creating an android application and I got a question about UX. I got a scrollable activity that will show some informations. The user can just validate or refuse the activity.

Something like that

Yes I'm not an artist

For validate or refuse the user have to enter a comment about why he chooses to validate or refuse.

What's the best option to do this ?

  • first, I chose to make a popup appear on button click, this popup contains fields that allows user to enter te information I need

  • second option would be to disable buttons on my activity while user does not fill a comment section at bottom of my activity

  • finally third option would be to refuse form submission if field is not filled so that when clicking button user will stay on activity if the comment section is empty.

I really don't know what's the most user-friendly solution, so if anyone could help me then it would be much appreciated as I really need to find the best solution there

Thank you all for reading me and my english needs a few improvements

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