What happens if I omit the "yang"?
Same meaning. My Indo wife says it doesn't matter which version is used.
Well, the connotations are different, though. Compare: What is the one that is red? & What is red?.
I think the only difference is: formal & informal usuage. Duolingo probably teaches only the formal variatn.
Can i say "siapa apakah menjual?" instead of "siapa yang menjual?"