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)."
Would tôi là cô gái be grammatically correct though? Or would that mean "I am THE girl"?
"gái" standing alone can be a negative way to address a woman/girl. It emphasizes the inferiority of gender role of women when a man uses this word. Additionally, "gái" can mean prostitute girl.
It's actually nice how words are put together like this since it's really easy to know how to be rude to someone on accident. [21/03/2019]
That would be "Tôi là một be gái.
Vietnamese's tones seem to be easier for me to distinguish by ear than Mandarin Chinese's tones.
For me it's the opposite but I interact almost always with southern Vietnamese and online courses are almost all northern Vietnamese. In any case I know I can't reliably distinguish the two falling tones no matter which dialect.