"Cậu bé dũng cảm ngã."
Translation:The brave boy falls.
That is a correct translation. In fact, both provided solutions for this question is wrong. Using "dũng cảm" as the adjective "brave" for "the brave boy falls" make the Vietnamese sentence becomes nearly meaningless. Translating it as a verb make the English sentence grammatically incorrect and meaningless as well. The only correct usage for "dũng cảm" in both Vietnamese and English for this sentence is as an adverb, so the sentence should become "the boy/little boy bravely falls."
"Bravely" may be the only correct translation of "dũng cảm" in this sentence on the basis of the Vietnamese but "the brave boy falls" is perfectly grammatical (if somewhat odd) in English. Given the position of "dũng cảm" in the sentence, it is difficult for a learner to know whether it modifies the noun or the verb. Both seem possible.