"Sự bất cẩn"
DL insists on "the carelessness" (on the smart phone version) which is simply wrong in English but I can't get out of the exercise without caving to the program. "Carelessness" is correct.
They are not the same at all. The morpheme bất comes from the Chinese 不 and is used in negative (negation-wise) Sino-Vietnamese terms like bất lợi (不利, disadvantageous/unfavourable) and bất tiện (不便, inconvenient). On the other hand, mất is a native Vietnamese word meaning 'to lose' ('to die' even).