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.
[ ма́ма → ма́му ] ‧ [ Nominative → Accusative ]
Nominative: [ Она́ ма́ма - She is mom ] [Mа́ма здесь - Mom is here ]
Accusative: [ Я вижу маму - I see mom. ] [ Я хочу воду - I want water. ]
Mom is back. She is happy. Have you seen her? Have you seen Mom?
Pronouns [ She ] and [ her ] refer to [ Mom ]
Pronoun [ She ] is in Nominative Case, the Sentence Pronoun Subject.
Pronoun [ her ] is Accusative Case, the Direct Object of the Transitive Verb, [ To see | have seen ]
‧ Accusative Case - Direct Object ‧ Whenever a verb, like "read", "cut" or "want" acts directly on some noun, the latter is a Direct Object. Such Nouns take the Accusative case. ‧ Formation - Feminine nouns ending in -а / -я have a separate form. «Мама» is a good example ‧ ма́ма → ма́му ‧ www.duolingo.com/skill/ru/Accusative-Case%3A-the-direct-object/tips-and-notes ‧
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.