In this case, the apple is the object of the verb. Masculine nouns (der) change to 'den' (or 'ein' to 'einen') when the noun is no longer the subject but the object. This sentence states that the apple is being seen. Meaning the apple is the object that the subject is looking at.
"You are viewing (as in looking at, observing) the apple" was disallowed. I don't think Duolingo's programming fully appreciates/accommodates the extraordinarily large, broad & diverse Wortschatz ( literally "word treasure") of the English tongue. There are many, many ways to express nearly every thought in English, and Duolingo falsely marks many acceptable English renderings as incorrect.