Eventually, I use waitForKeyElements and it works.
My current script: Dropbox - blacklist-stable-0.1.2.js - Simplify your life
I am trying to create “Looking for a different answer”, and I now require [Userscript] Prevent "Your answer was a bit off" answers from being accepted a.k.a. "Close But No Cigar" to work with (Obsolete) [UserScript] WaniKani Double Check - #72 by rfindley and also with my script… because I feel my script looks partly like No Cigar (and probably partly like [Userscript] Wanikani Multiple Answer Input in concept).
About MutationObserver and observer(), the answer must be javascript - Mutation Observer for creating new elements - Stack Overflow