Translation:The green dresses are hers, and all of them are beautiful.
Ha-smalot ha-yerukot hen she'la, ve'chulan¹ yafot.
¹ from pages 65-66 of "Essential Hebrew Grammar" by Lewis Glinert:
The Hebrew of broadcasters, teachers, and their ilk makes certain rather complicated adjustments in pronouncing words beginning with a prefixed ל, כ, ב, & ו. Colloquial Hebrew generally doesn't bother.
(The rule in this case, I think would be: "if the next letter is כ, ב, or פ, it will be soft. This is also explained here: https://www.quora.com/How-do-you-say-and-in-Hebrew/answer/Marta-Krzeminska )
See my previous comment. It's a fine translation.
The designers of this sentence were probably looking for a structure that directly parallels the Hebrew, with כולן translating directly to "all of them". (Either that, or they just didn't get around to coding multiple correct answers.)
You need the copula הן to take the place of the verb "to be", which doesn't exist in the present tense in Hebrew. Without the copula, the first clause would mean "her green dresses" rather than "the green dresses ARE hers".
The general rules for when you need a copula and when you don't are found in one of the first Tips and Notes on adjectives. However, there seem to be a lot of exceptions to those rules and a lot of subtle details on when to use הוא or היא or הם or הן versus זה or זאת or אלה.
Here's one more subtlety: I'm a native speaker, and omitting הן here sounds only a bit less natural than with הן, and quite OK. Note that in speaking, the intonation would usually make it clear that you are not saying "her dresses", and if not - then the fact that you ended the sentence right there would clarify it...
chanieHoff wrote that the word יפה means both pretty and beautiful, and that both of those words should be counted as correct in this sentence, which they are not. In some sentences both are accepted, but here "pretty" is not accepted. That was the point of the post.
And then you wrote that יופי is "beauty", which is correct, but irrelevant to their post, as "beauty" יופי is a noun and they were talking about "beautiful" יפה the adjective.