"Delfinii mănâncă pești mici."
Translation:The dolphins eat small fish.
That seems to be the default translation. But your emphasis on "the" suggests you had to translate "Dolphins eat small fish". That is a general statement about all dolphins and the way to express that in Romanian is using the definite article. So "delfinii mănâncă pești mici" can mean either "the dolphins (some particular dolphins) eat small fish" or "dolphins (all of them, in general) eat small fish".
Also, since Romanian apparently has only one present tense (correct if I am wrong) another possible translation is "The dolphins are eating small fish." It is accepted by Duolingo as correct.