"I'll be back."
Why does the tool-tip glossary give no hint at all of the acceptable answer? Instead it gives three words that do not appear in any form among the possible answers. I quit.
Honestly, my clue to the correct answer was simply the future form of the verb. I believe there was only one future form. I thought, "it has to be this one!" Don't quit now. Korean is very difficult. Just keep trying.