The sound is not correct for ""sự bất cẩn" but it's true for "sự bất cần", because these are two different meanings.
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.
So sự cẩn is the opposite? Cautiousness/attentiveness or something like it?
Am i correct that mất and bất are the same thing? Is this a southern- northern thing?
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).