The passing year on your marksheet after appearing for a supplementary exam as a private candidate will be the current year.The purpose of a supplementary exam is to clear a backlog subject from a previous year. When you pass the supplementary exam, you're essentially fulfilling the requirement for that subject in the current academic year. Appearing as a private candidate doesn't change the fact that you're taking the exam in the current year. You're assessed based on the current curriculum and standards.

Therefore, your marksheet will reflect the current year as your passing year, indicating that you have successfully cleared the subject.

I hope it helps !