It says one of the answers is "who tell you?" which is incorrect English. I put "who told you", but if you want to keep it present tense, "who tells you?" would be better.
it can only be plural with a referent. "Who are the people that are going by train?" You CANNOT say "Who are going by train?" You can, however, ask "Who all are going by train?" All is the referent to the plural.
'Who are going by train' sounds fine to me. 'Who all' is pretty much strictly southern US dialect.
Why not với bạn? The last sentence used this with notifies and now we have cho. Next time I have a similar sentence, I will use cho and get marked wrong. Is this course an experiment to make people schizophrenic?
Với: I want to notify her. The teacher notifies my parents.
Cho: who notifies you?
Songve Rule: Use cho for question rather than với. This is the only rule I know unless or until someone comes up with something better.