Eple is neuter so its gets et and not en.
I have some confusion with the article et. If it can be et eple then why not et bok or et avis. The latter two are marked incorrect
Because in norwegian, the nouns have different classes: masculine, feminine and neuter. "Et" can only be used for neuter nouns, or you will sound completely weird.
So is a rule for this classification? How does one deduce that a noun is neuter or masculine/feminine?
You should memorize it's class when you learn it (don't just learn "eple", learn "et eple").