All the options are incorrect, if you're talking about circle! which is most common C = 2πr..... It is the relation for Circle's if you're considering something else, then, please mention!
The two points can be made into a right triangle with the two sides defined by two lines drawn from the two given points. Use the Pythagorean Theorem to solve for the hypotenuse, the distance between the two points. See attached graph.
You could expand terms out and simplify, but I think it's more handy to leave it in this form so you can easily spot the center and radius from a glance.