Colloquially "who" might be ok, but learning a language surely that should be "whom"
But if you are learning a language with the purpose of communicating, doesn't it make more sense to learn the way it is actually used?
Isn't this course for people to learn Vietnamese, not English?
Anyway, "whom" might be correct here, but I haven't met anyone who spoke like that in the real world unless they were being pedantic.
"Whom" is correct. There are people who use it correctly and it is understood by all native speakers. Duo should at least accept it.