This was an iOS app question, where the available prepositions were "at" and "in"; "of" was not an option. I think "at" and "in" are both suboptimal, and it is best translated with "member of", but I had to guess and was marked wrong for choosing "in". I wish reports could be made from iOS!
Of course I know it sounds unnatural without an article in english...
But there are so many times you mark my answer wrong because I subconsciously added a 'the' to make a sentence more natural.
Why don't you just prompt a "Other correct answer:" in this occasion?