Translation:My friend wants to buy a bag of apples.
They are very similar and we don't differ these two classifiers too much, but 一袋 sounds like the bag is opened, like a plastic bag, and 一包 is used more for a sealed bag, like chips. Sorry that my English is not good enough to explain this well, and I believe this difference depend on regions, or even depend on speakers, so you can just take this as a reference :)
That would be "My friend will buy a bag of apples.", which I think is a technically correct interpretation. I've no idea if it's accepted though.
("Going" is progressive tense, which hasn't been taught yet, but Duolingo has allowed that in several other exercises while rejecting it in others, so I'm not sure what should be accepted.)
Another ridiculously narrow choice in answers
It won't take "My friend wants to buy one bag of apples". I've always written "a bag of apples" but I thought I'd try the other alternative. This one is absurd because it accepts "one bag of" in many other examples throughout the questions. Yi, or 一 means one.