this is a common question in the opposite course for czechs.
i suspect you answered "pants or dress". for the czech side to make sense, "šaty" was not used as a mass noun (clothing/clothes). (Are you buying pants or clothes? is just off, like apples or fruit.)
so it will be a countable noun in english, and thus either "a dress" or "dresses". as you must know, duo keeps offering the nearest acceptable alternative, and the distance from "dress" to "dresses" looks shorter to it than that to "a dress". priority on matching the number of words maybe?