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Qxg7
Checkmate is achieved by White, who uses his Queen to capture (x) Black's Bishop on g7.
White's Queen now guards all light squares (f7 and g6), plus all the dark squares (e7, e5, f6 and g5), which prevents the King making any escape within his legal range of movement.
Other White pieces, guarding exit squares, include the Bishop (covering f7 and g6); and the King (protecting g5).
Ideally, Black would dearly like to get his King out of Check by capturing White's Queen ...
But, this isn't possible, as White's Queen is being protected by the kNight, on f5.
With no other Black pieces able to get across to deal with White's Queen, it's "Checkmate".
White Wins.
