In the "Tips & notes" above this exercise it is written:
"In Vietnamese sentence, if you see the word một, then your English answer should contain a/an. If not, then your English answer should contain the. In English sentence, if you see a/an, then your Vietnamese answer should contain một. If you see the instead, then your Vietnamese answer should not have anything before the noun (except for classifiers which will be taught later)."
The northern dialect is far more common with online resources. If you want to learn the southern dialect, YouTube can be a good resource. The most helpful thing would be to find a native speaker who can help you (most Vietnamese people living outside Vietnam speak the southern dialect, or at least close to it).