Is it offense or offence ?
That depends upon whether you are using British or American spelling systems (both equally valid).
There are several instances in British English where words such as licence, practice, etc are spelt with a 'c' when used as a noun, but with an 's' when used as a verb. For example:
"We went to rugby practice, where we practised scrums and lineouts."
"His driving licence shows that he is licensed to drive HGVs."
Of course the verb from offence is offend, but the same principle applies - US English always prefers the 's' spelling over the 'c', and makes no distinction between the spelling of the noun and the verb.