slingfox
My biggest problem is the wording of the question. The question asks for minimum number of applies she must buy to ENSURE that the total number at the end of the day is a multiple of 5. The explanation, however, speaks of the minimum number of applies that need to be bought at the convenience store so that she CAN end up with a multiple of five apples. CAN and ENSURE mean very different things, so I'm not sure why A is correct.
Look at it this way: I want to
ensure that I have 3 chocolate bars. store A only sells 2 at a time so I
need to buy 1 from store B so that I
can end up with 3 chocolate bars.
It just means that this is what I must do/need to do/must ensure so that I can make it a possibility. Don't worry too much about it.
And it seems the question is simply testing whether you understand that 0 is the smallest non negative multiple of 5. If you buy 0 apples, you buy none from anywhere! It would have been different if it was given that she must buy some apples.