I replied with 'shop' instead of 'store' and this was accepted - however, this leads me to wondering if there is a separate, specific word for 'shop' as opposed to 'store'? In 'British' English they do have slightly different meanings/connotations. 'Store' I would generally take to mean a large shop, like a supermarket, whereas 'shop' can apply to places of any size.
The choice of i and på for locations for various usage is a difficult part in Swedish, since there are no real rules. There are a bunch of guidlines, but many exceptions. One kind of has to learn by experience and practise. There are also a few regional differences.
For the phrase "Han arbetar på/i en/ett (workplace)" one can often use either one, with no or little difference. There are some cases though where one of them sounds odd or wrong, and the other should preferably be used.
Use i for: en affär & en butik (store, shop, boutique), en gruva (mine).
Use på for: ett hotell, ett universitet, en gård (farm), ett kontor (office) and when using the name of a company.
There are more, at least of those that want på, but these are some that came to mind. Examples where both i/på works are: en stormarknad (supermarket), en fabrik (factory), ett apotek (farmacy), ett bibliotek (library), en skola (school), en verkstad (workshop), en bank.
Note however that when saying e.g "Han är på banken" you should not use "i". One could say that the bank emploees are allowed inside, while visitors must stay by the counter.
Hmm, if a friend said 'I'm going to the store; do you need anything?' I would know she was going to a grocery store, specifically, and to no other kind of store. However, If she told me that her uncle worked in 'a store' I'd have to ask her what kind: a shoe store, grocery store, department store, etc. I suggest that the multiple choice answer word (store) be changed to 'grocery store' since that is apparently the kind of store an affär is. Otherwise, I think this answer is a bit too vague.