The correct answer is: Dresses

The correct answer is 'dresses".
Here, we have more than one dress, so it is plural number. The most common rule for changing any word from singular to plural number is adding "-s" or "-es" at the end of that word. We must add "-es" after those words which end with "ss","s","ch","sh","o", and "x" to make them plural. Thus, we must add "-es" at the end of the word "dress" because it ends with "ss". So, the correct answer is "dresses".