The use of adverbs in english

I just realized that I need to get help with understanding how to use adverbs in English. Let's say that a guy is nice, and he performs every action in that way.

Does he:
a) act nice
b) act nicely

Which one is the correct one and why is it so? Don't be afraid to use grammatical terms.

>>1
Being prescriptive, "act nice" is incorrect - "nice" is an adjective, it modifies substantives, not verbs. In this case, use "nicely".

Being descriptive, yes, -ly particle will fall hard in the next centuries. In 23th century Britanic, Merkinish and Aussish, probably "act nice" will be correct, since "nice" will work both as adjective and adverb.