The verb is conjugated in the plural. Ze eet is singular (she eats), ze eten is plural (they eat) http://www.verbix.com/webverbix/go.php?D1=24&T1=eten&H1=124
Yes, so next time that you get that question, please report that your answer should be accepted as well.
Note that this is about English; Dutch doesn't have two tenses for present - a lot of languages don't - thus wondering which English present tense to use doesn't actually improve your Dutch much. Just stick with the simple present, like Dutch does, and report that your solution should be accepted, on any occasion where the course doesn't as yet accept it.
A lot of languages do this. German uses "sie" for "she" and "they" too. Italian uses "sono" for "I am" and "they are". English uses "you" for "single you" and "plural you", which I'm sure many learners find annoying.
Languages often just develop in unexpected ways. There was no meeting where Dutch speakers collectively decided to "use ze for both they and she".
I understand very well that "ze" (NL) is used for "she" and "they" (EN). And when they are written down, I can very easily distinguish between "ze eet" and "ze eten". However, I have heard that native speakers of Dutch tend to omit "n" if it's at the end of the word. When I listen to this, all I hear is "Ze eet rijst". Does anyone else have this problem? Do "Ze eet rijst" and "Ze eten rijst" sound the same when spoken quickly by a native?