On Wedding RSVP card will it be appropriate to mark Attending 0 box or Regretfully Decline box if not attend?

Question

My niece is getting married in Hawaii leaving in Chicago.

We were planning to go but we are Back To School & it's in Sept. second week right after school reopens so can't attend.

Answer 1

Regrettfully decline and put in a little personal note.

Answer 2

Regrettfully Decline.

The attendance box is just for people are are going and to mark how many people will be attending.

Answer 3

regretfully decline- and add a nice note to congratulate them