"The boy's apple is food."
Translation:남자아이의 사과는 음식입니다.
I believe this type of question is trying to make people figure out the answer based on the context, if you know 사과는 음식입니다 then it was reasonable to infer this was the answer compared to the others. You will know always know the vocabulary but you can try to use what you know to reveal the clues.
Actually, 의 has three sounds - [의], [에] and [이]. Not sure how can you determine what to pronounce, but I saw an example like this: 민주의 의의 (Significance of democracy), but it is pronounced as [민주에 의이]. The first 의 acts as 's, following by 의의, which is "meaning, significance etc.". Hope this helps you.
they introduced another word for girl (other than the one they taught us) in an earlier practice, so here they are doing the same thing. IF google translator can be trusted then 소년의 is indeed another word for boy. I think we can be reasonably assured this may be correct as "sonyeon" is part of : Bangtan Sonyeondan ( bullet proof boy scouts , A.K.A. :BTS) ! To throw a word at us like this without any introduction seems a bit rude to me.