"Give your grandmother an apple."
Translation:할머니께 사과를 드리세요.
I think both should be accepted. Since it's a command sentence, "your" is basically implied, since you're not gonna share your precious, fresh-picked apples with someone else's grandma!
But also it's common to call her "grandma" as a name, eschewing the need for the possessive pronoun.
I feel that the same idea applies in both languages here. In English, we also commonly call our own parents and grandparents by their title in place of a name, and use "your" or "my" for clarification. I feel the English translation should indeed be "Give grandmother..." instead of "Give your grandmother..." or the Korean answer should have included a translation for the "your." I'm sure by the end we will learn it both ways regardless.
There is a real problem of consistency everywhere... sometimes Duolinguo accepts NOUN+에게 sometimes it accepts NOUN + 께 (when you give something TO someone for exemple). Sometimes both are accepted, sometimes only one... seems to be zero rule and just keeps convincing me that I just should never go for the payed version of duolinguo