As it has been already said, Apfel is masculine and Orange is feminine. They both are in Akkusativ form, but for the feminine, the nominativ and the akkusativ form are the same (eine). On the other hand, the Akkusativ form for maskulin (einen) is different from the nominativ (ein). Hope this helped!
I'm really confused about the pronunciation of "Orange". Is the "e" at the end of this word silent or not? It is pronounced in this example, but in some other cases, like in the dictionary, it is not: https://www.duolingo.com/dictionary/German/orange-Noun/01504dd8e9ed5e96be568302cb1ac709