"Who studies Korean?"
Translation:누가 한국어를 공부해요?
누가 is a contraction of 누구 and the subject particle 가.
So, instead of 누구가, it becomes 누가 when "who" is the subject of the sentence.
I would say it's because this particular lesson is about moods in 해요체 and the (스)ㅂ니까 ending is a part of 합쇼체. In 해요체 the verb form does not change from the declarative form.