there isn't one... but if there was, it would be C.
On the other hand, the rule of thumb is that an acronym form a pronouncable word, as opposed to an abbreviation. For example, DLL is a abbreviation(For Dynamic Link Library), since it doesn't form a pronouncable word (and is usually pronounced Dee-Ell-Ell). On the other hand, BIOS, which stands for "Basic Input/Output System", is an acronym since it can be pronounced "Bye-oss" or "bye-ose" without breaking the strict rules of pronounciation. Which variant one uses becomes one of preference and perhaps their native vernacular, similar to po-tay-toes versus pah-Tah-Toes type thing (and likewise for the good ol' nightshade vegetable).
