Hello ouangel,
To do that you need to use Manual Adjustment > Regional.
1. Use the
adjustment selection tool to select/mask the area/object you want to keep in colour (in this case, the duck)
2. Click on
Invert Mask - which selects the area outside the object (duck)
3. Make whatever adjustments you wish to the area - saturation to -100
PIX
PIX YouTube channel