Final answer:
The duck with a green head and a chestnut breast separated by a white ring is the Mallard.
Step-by-step explanation:
The duck described with a green head and a chestnut lower back and breast area separated by a white ring is the Mallard (Anas platyrhynchos). This species is recognizable by these distinct colors and is one of the most common and widespread ducks in the Northern Hemisphere. The other options provided do not match this description. The Northern Pintail, Canvasback, and American Black Duck each have their unique plumage that does not include the combination of a green head and chestnut as noted in the question.