What is the full form of DC in DC comics?

DC means simply "DC," as the first D in D-Day is just "D."

The initials come from DC's first regular publication, Detective Comics, which eventually became home to Batman. So there's a case to be made that "DC Comics" is redundant. (ref: Mickey Blue-Eyes)

That's the source of the initials, but the full form of the company name (which is now a partnership owned by Warner Bros so it no longer includes "Inc") is "DC Comics."