The standalone vocab word (purple background) is つき. The kanji question (pink background) wants you to type either げつ or がつ.
The kanji on its own will always be read つき when you come across it in the wild (unless it’s an abbreviation for 月曜日 like you might see on a calendar).
In compound words, it can be one of all three readings, and sometimes even two readings are acceptable, though one will be more common.
I was trying to find a post I wrote on this once, but couldn’t find it. This thread from a while ago might be of help: