I don't know the lesson (most people answering here are not Duolingo employees but volunteers or other learners).
1) Orange is feminine: eine/die Orange (sgl.); --/die Orangen (pl.)
2) However, "ein paar" (a few, some, a couple of) is a fixed expression. It never changes, irrespective of the gender and case of the noun. It has nothing to do with "Orange" or "Paar" being masculine (which they aren't). E.g. "Das sind ein paar Orangen/Männer/Häuser" (nominative), "mit ein paar Orangen/Männern/Häusern" (dative), etc.
3) Etymologically, the use of "ein" can probably be explained by the fact that the expression "ein paar" is related to the neuter noun "ein Paar" (a pair, a couple). But this is only a historical explanation - in the fixed expression "ein paar", "paar" is not a noun and the article "ein" doesn't act like a normal article since it doesn't change in the different cases. it's just a fixed expression.
4) Note the difference in spelling:
ein paar = a few, some, a couple of
ein Paar = a pair, i.e. two
You mark in the lesson about food the word orange in german as femine -to be used with Die-, yet in the correction to this question you state it should be used with Ein -and not einE- as it masculine.
Isn't there a mistake here ? moreover, the "ein" should follow the genre of "Paar" rather than "Orangen".
Thanks a bunch for your answer and for this website, a cheap, yet efficient way of brushing up my -very- rusty german.