Did you find it? A mate in 1. The answer is: simply promote your pawn to queen. Mate!
Why is it mate? Well, if you would try to take the queen with the knight on d3, then the 3rd row would shrink, and the kings would be adjacent - which is not allowed.
Then, you might think that you can take the queen with your king. However, the queen is clearly covered by the bishop! For, if you would take the queen, then the second row would shrink away, and the bishop would attack the king.
No other move removes the king from check, so that's the definition of mate.