It's legit. It's not a complete sentence with verbs, but it is simply describing a particular watch. Which watch are we talking about? The watch in the purse.
In previous cases where it was not a sentence, it did not end in a period. Also, the Vietnamese sentence here is a full sentence since ở is a verb "to be at a place", so there's no reason for the English sentence to not also be a full sentence.
I see what you're saying. I see the period now. The best correct answer would be a full sentence. Regardless, to say the phrase "the watch in the purse", we would say "chiếc đồng hồ ở trong cái ví". I just wanted to point out that "ở trong" can also just mean "in". Sorry for not seeing the period and I hope people can understand that a sentence like this can also be used as a phrase in Vietnamese =)
On android the English sentence to translate appeared as "The watch in the wallet". I've done the lesson again but this time the Vietnamese sentence appeared where you have to select the English words in order. Similarly I was able to enter "The watch in the purse" as a correct solution - this shouldn't be correct. I'd upload a screenshot but I don't know where to put it!
Will ở always be used without là to represent is/was/will be [preposition]? Any time they'll be used together?
I'm trying to think of any instances where they're used together, but I came up with nothing. I'm pretty sure you just use "ở".