"We eat his rice."
Translation:Nous mangeons son riz.
In this program it seems to go like this:
I eat rice -> Je mange du riz (Also equivalent to I eat (some) rice)
I eat the rice -> Je mange le riz
But in this question, it's his rice that we're eating meaning the rice belongs to someone. But whether we use 'son', 'sa' or 'ses' depends on the gender of the rice, and as rice in french is 'le' riz and NOT 'la' riz, we use 'son'.
I eat his rice -> Je mange son riz
I eat her rice -> Je mange son riz (the same as it is 'riz' that we look at the gender of)
I hope that helps :P
mangent --> Ils, Elles; Mangeons --> We; https://forum.duolingo.com/comment/13396/Mange-manges-mangent-mangeons