The website (or that chess problem on that site specifically) is clearly buggy.
All 3 of the moves listed as correct answers are mates, and even shown with #
Qe7# Qc8# Qb8#
My guess as to what happened:
- The web site supports various different kinds of problems, including "one correct move".
- It doesn't have a chess engine checking for mates, just that they match some pre-programmed answers. It relies on humans to set up the correct-answer conditions separately from the question and answer text. (Introducing the possibility of human error causing this problem.)
- A human entered correct problem and answer text, but accidentally marked it as only the first answer being correct. Or some kind of syntax error or other mis-use of the problem-setting tools that made the website not understand that any of the answers were valid.
That would explain why all 3 listed moves have a
# checkmate symbol after them: those are manually-entered text, and the web site doesn't truly know they're checkmates. Or doesn't know that they question is looking for checkmates, because programming that is separate from writing the text.
In future, if you encounter something online that seems to be telling you something that seems wrong, you can load up the position in a chess engine you trust and try it. Maybe you'll find there was a legal move you didn't spot.
As a beginner (or for any human) it's certainly possible to make mistakes. But this case is simple enough that it's pretty easy even for me (very much non-expert) to be pretty sure that all 3 of those moves are mates, and that there aren't any other mates. But it wouldn't take much more complexity for me to not be sure if a problem was buggy or if I was missing something.
In this case, the 3 listed moves that pop up being shown with
# makes it fairly clear it's a bug, though.