See all conjugations, see this link: http://cooljugator.com/ru/%D0%B2%D0%B8%D0%B4%D0%B5%D1%82%D1%8C
The grammar of the sentence is: Subject noun (nominative) -> predicate (verb conjugated to match subject) -> direct object (accusative). Since "mama" is a feminine word with a normal feminine ending (-a/-ya), in accusative case it will take the -y ending. Russian grammar is tricky at first but it's all very logical. If the sentence were "Devochka vidit mama", it would not be clear at all who was seeing whom.
In English, "The girl sees mom" most often would be interpreted to mean "my/our mom"; the sentence construction turn the word "mom" into a proper noun/name (it should be capitalized to Mom if that is the case) rather than a descriptive noun (where it should not be capitalized).
That brings up a question: Names in Russian appear to be all capitalized, so are all proper names also capitalized, like in English? That would go a long way towards setting the context to clarify what or who "mom" is.
I was expecting «Девочка видет маму.» for «The girl sees mother.» but, according to the conjugations that I looked up according to diogogomez's link, the «Девочка видит маму.» seems to be meant for past-tense into «The girl saw mother.» and as a result I was thinking that видит was the plural-conjugation since I am not mastered in all of the conjugations (causing me to think it might be: «The girls see mother» and it marked my answer as incorrect).
Because the answer is incorrect... Девочка can only be a singular noun (plural would have been девочки). In Russian, most verbs with a regular root just drop the ending and adopt an -л / -ла / -ло ending for past tense (видел / видела / видело). If you're using an app or phone browser I recommend checking out that conjugation page again on a desktop or laptop, as it very clearly lays out the conjugations in present tense right underneath the main chart, and past tense conjugations right underneath the present tense list.
Sometimes the Duolingo algorithm gets confused and gives you a weird answer. When that happens, I sometimes copy the "correct" answer into a text file and then copy/paste it into the answer slot the next time the sentence comes around. Every time that I can remember, Duo marks its own weird answers as "wrong". When you get a really odd "correct" answer, do not trust it - it's probably wrong, too.