I wondered about this too, on a different exercise - but on looking it up I can't find a dictionary that translates 'costume' as 'outfit' - I guess there's a subtle difference between the words (though to me the two - in English at least - are roughly similar)
In my experience, Italian "il costume" can often mean "bathing suit" or "swimsuit." The fact that it doesn't immediately offer "swimsuit" as translation help had been bothering me all through the clothing unit. I finally just tried "swimsuit" on this sentence and it took that as correct. Technically it's "costume da bagno," but I do remember hearing Italians at a beach in Italy using simply "costume" to mean "swimsuit" since it contextually made sense.
Completely agree D_Lauren, my family and friends use "costume" for "bathers" or "swimsuit". I was also surprised that this wasn't offered in the help as in every day conversation I would use this term more often.
It may help. In German we refere to a custume as a suit, jacket and pants or skirt matching.
In America, "costume" usually has a narrow meaning, indicating clothing one would wear to masquerade as someone or something else. Thus, actors wear costumes; children wear costumes on Halloween (as they "dress up" as ghosts or witches or pop culture characters.) It is not applied to everyday wear and, usually, not to swim suits, technician uniforms, etc. So...what is the best Italian word for the American "costume?"