The answer is dxe6# en passant. However, you did not specify that en passant was legal. Also, white has two pairs of doubled pawns, which means white has made at least two captures. But black has all pieces and 7 pawns, so white could only have made one capture. Therefore, this position is illegal.
Hardest Checkmate in 1
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Jul 1, 2021
0
#145
Th tt5yu77777777777777777777777777777777777777 777777777777777877777777777777777777777777777777777777777 7777777777778777777777
Jul 1, 2021
0
#146
srikar0007 wrote:
Th tt5yu77777777777777777777777777777777777777 777777777777777877777777777777777777777777777777777777777 7777777777778777777777
Qf6+ is a mate in 4.
1. Qf6+ Kxh6 2. Qh4+ Kg7 3. Rbxf7+ Kh8 4. Qxh7
The best I can see is mate in 2.
1. Rfxf7+ Kh8 2. Rxh7#
Jul 2, 2021
0
#148
Datta_Vamsi_Krishna wrote:
Qf6+ is a mate in 4.
1. Qf6+ Kxh6 2. Qh4+ Kg7 3. Rbxf7+ Kh8 4. Qxh7
The best I can see is mate in 2.
1. Rfxf7+ Kh8 2. Rxh7#
En passant 
Jul 19, 2021
0
#150
Rsava wrote:
chiefonion wrote:
The king can take the knight
The hardest mate in 1? Really?
Qf6+ is a mate in 4.
1. Qf6+ Kxh6 2. Qh4+ Kg7 3. Rbxf7+ Kh8 4. Qxh7
Best I can see is mate in 2.
1. Rfxf7+ Kh8 2. Rxh7#
(And @chiefonion changed his post after I started mine.)
mate in one
white Qf6
It is probably dxe6# although it didn't mention that black's last move was e5. It's a good en-passant puzzle by the way. I like it.
its obviously Qf6#
It will be nice to see the last move played by opponent how else are you supposed to know its en passant?
It's a pity the given starting position is illegal to begin with.
Illegal how?
2 sets of doubled pawns for white. 1 captured pawn and nothing more for each side.