* No punctuation: INC can be written without any punctuation. This is the most common practice, especially in informal contexts.
* Period after each letter: INC. This is another common practice, especially in formal contexts.
* Period after the last letter: InC. This is less common, but it can be used in some cases.
Ultimately, the best way to punctuate INC depends on the specific context and the style guide you are following.