All those "checkmates in 1" are easy. This one is hard. As you can see it's a chess960 diagram. And oh, it's not 1. c5-c6; figure out why.
It's probably Black to move.
I did consider the king coming from e7, until I realised that both the knight and the e6 rook were both attacking the e7 square at the same time.
And that bishop on h8 cannot have been promoted.
If it was black to move, it can't checkmate.
Wait a minute. If it's 960 as per the heading, the last move could have been castling. Hang on...no, that position would have been the position after castling, so it's not possible.
With the given position being White to move on standard chess, it would not have been possible to reach that position. En passant or regular capture cxb6 would be the checkmating move.
It says nowhere that "white is on move". The standard agreement is that white is on move if possible, otherwise black moves first. That is where the chess960 castling option comes in. Still white gets 1 move to execute his checkmate.
None of this is unusual, it even applies to endgame studies. Black starts if white's start can be disproved! Of course, black may have many different options to start play and white needs to solve the challenge for all of them!
Note that "n moves" in chess communication commonly refers to "about" double that amount of single moves. In compositions it always specifies "the number of white moves", leaving the number of black moves up to the type of challenge or the conditions. Some challenges use half-move notations like "5.5 moves" indicating precisely 6 white and 5 black moves but that is just a refinement.