I-539 Applicant Signature

[sudbon2784]

I am applying B2 extension for my parents. My father got stroke a year ago and his right hand and leg were paralyzed. Now he is partially recovered, but he is not able to do a signature in I-539 (Part 6.a.). I have a doctor note stating the incident happened last year. Please suggest me which options would be better -

1. Place his right index finger print on the application and include explanation in "Part 9: Additional Information sheet"
2. Let him sign the way he can. But it will not look the same as it was in his passport. Do I need to notarize this page?
3. I will sign on his behalf and include explanation in "Part 9: Additional Information Sheet"

Thank you for your inputs.