That's the distance formula, which is the Pythagorean Theorem applied to the points.
The distance between (a,b) and (c,d) is
a-c is the signed distance in the x direction between the points. b-d is the signed distance in the y direction between the points. Since the axes are perpendicular, these make a right triangle whose hypotenuse is the distance between the points.
Here that just means our distance is
Answer: B 4.1 units