"They are fine."
Translation:Ellos están bien.
It is ok, but it has a slightly different meaning: They are fine in the sense that they are always or intrinsically fine, such as good-looking.
Because ellos is a verb from estar, and estar is used to tell feelings. They are feeling well.
ser and ester seems to be used arbitrarily. Ellas estan bien but soy enfermo???
Soy enfermo says I am a sickly person, always sick. Estan bien means they are doing well today, but not necessarily all the time. The meanings are not arbitrary -- but Duo may be. ;)
I may be wrong but I think it's a contextual thing; 'ellos son buenos' means they are well in a general sense, depending on the context, if they weren't doing well and are now well 'están bien' would be more appropriate (i think the english translation of 'fine' implies this...)
Bien is an adverb (modifies a verb) and bueno is an adjective (modifies a noun).
This was my answer selected from three choices. My answer is the same, the first selection on the list, yet I was given an incorrect by the machine?
I think without context, either "ellos están bien" or "ellos son buenos" should be accepted. If you were talking about some shoes, wouldn't "ellos son buenos" be correct?