Parentheses and slashes make the area code look optional. Since most major metropolitan areas have moved to 10-digit local phone numbers, making the area code a mandatory element of dialing any number, I use dashes. It looks very European and dot-com. At first I was derided for preferring this European format, but I am noticing its increased usage everywhere. Other choices look a phony attempt to be hip.
C looks like an Internet address for a server.
We use a lot of san serif font and I find that the slash very difficult to discern from some numerals in san serif. The hyphen just seems to provide more space without cluttering the number as the slashes seem to do. This one is all a matter of style and is dependant upon the image that the company wants to present. I also like the third choice, but I struggle to remember to use it. All are correct, so it is merely a stylistic preference.