"which is black"
I learned that Melyik means "which" I was wondering why "ami" was chosen here and not "melyik"
The use of all lowercase here is supposed to suggest that we're looking at a sentence fragment, not a complete sentence, so "which" is the relative pronoun (like in "the cat which is black") rather than the interrogative ("Which cat is missing?")
(It's not a very good exercise.)
melyik is "which" as an adjective, e.g. before a noun. "Which cake do you want?"
ami (and amelyik) are relative pronouns, which stand by themselves and tie a relative clause to something in another clause. "I want the cake which is black".
This is not a whole sentence (not the absence of final full stop) and they seem to want to ask for the relative pronoun.
As for when to use ami and when amelyik as a relative pronoun, I'll leave that for a native speaker.
Why is: ami fekete VAN, wrong? Is it not possible to use VAN? The context is missing and how do we know, if it is needed or not?