N, N-dimethylformamide is the correct one.slayerdeus said:*n: for normal, meaning if you have a substituent group, usually higher than 4 in my dealings.
->example: it is common to call n-pentyl, n-hexyl, etc. instead of pentyl or hexyl which could be confusing; n is just used to easily identify the "normal" alkyl substituent
N:for amines, used to show what substituents are bonded to the nitrogen atom
if there are two attached to the nitrogen, then the above can be called dimethylformaldehyde or N, N-dimethylformamide
correct me if i'm wrong, hope that helps