"The athletes are slow and weak."
Translation:A sportolók lassúak és gyengék.
As "sportolók" and "atléták" are both words for "athletes", I think my answer: "Az atléták lassúak és gyengék" should have been accepted. I'll let the DuoLingo team know, and see if they agree that this can be an alternate translation.
"Atléták" may be more specifically used in Hungarian for people who practice the types of sports under the umbrella of "athletics". That is, track and field and such. The more generic, more encompassing, term is "sportolók". With that in mind, I think both could be accepted.