I have come up with a term on my own to describe the word that fills a fill-in-the-blank. I have named it a 'blank-filling word'. Can this be used? Is it proper to use this? I understand that it isn't an established phrase yet. But, will other people get it when they come across it?
It would be better to say fill-in-the-blank word. For example:
Please don't just give me a fill-in-the-blank word. I really want you to think about this sentence I'm trying to write, and come up with something well thought out.
Note, if you're just trying to talk about the word that goes in the blank, then just say "word." (If a phrase was requested, then "phrase.") For example:
I gave my students a fill-in-the-blank exercise today. I have to tell you the word my charming older gentleman student chose. I thought he had made it up but it turns out it is in the dictionary!
It depends whether the answer is opinion-based or if it is objectively correct/incorrect.
For example, if someone is completing an opinion-based survey, the "blank-filling word" might be described as a word that you feel best matches the prompt (or the sentence that contains the blank). However, if there is an objective correct or incorrect answer, the "blank-filling word" might simply be described as the correct answer.
These will change, of course, depending on whether the actual answer is a number, word, letter, etc.