"who is beautiful"
"who is beautiful" is not a complete sentence, and it is inappropriate to test
It also looks like you are asking a question but forgot to add the question mark.
This fragment alone would never be used in English.
Raphi, the DuoLingo phrase is perfectly appropriate. The fact that there is no capitalization and punctuation make it clear that the example is not a complete sentence. Duo tests us on individual words as well as sentences, so why not also on phrases that could be part of a sentence?
I disagree. Testing sentence fragments that stand on their own makes sense, especially very early in a course - e.g. for teaching gendered articles in German or the Romance languages it is common to teach just the article and a noun alone (die Frau, das Mädchen; el gato, la gata).
But the whole point of this phrase is that it is dependent on some other part of the sentence - the *, which is beautiful, does ****. Teaching the dependent phrase in the absence of its essential context is confusing rather than clarifying, because there is no meaning associated with it.
Could someone explain to me why it is "aki szép" and not just "ki szép"? Are they just interchangeable or do they convey different meanings or what? Please let me know and thanks.
"Ki" is "who", like in a question. "Who is beautiful?"
"Aki" is "the (person) who/that/which". "The girl who stands there..." - "A lány, aki ott áll ...".
Here, since this is not a question, "aki" seems to be the correct choice.