"The animals are drinking water."
Translation:Les animaux boivent de l'eau.
I think this should be de l'eau, not d'eau. "de" follows negation and expressions of quantity. Le, La and Les follows verbs of preference and du, de la and des follow all others. I do not this this is correct.
Maybe this'll help: http://french.stackexchange.com/questions/1556/usage-of-deau-vs-de-leau
Apparently this d'eau usage is very exceptional.
Even after reading an explanation of its use I still don't understand why it is used here.
In the example that I have seen d'eau is used to show possession by what the water is in. eg: a glass of water, a jug of water. The glass or jug possesses the water.
There is nothing in the Duo multiple choice example to show possession by a container, pond, stream, lake etc.