It is A because the radius is 10 which means that if x=0 then y must equal 10. (0,5) would be inside of the circle. If this doesn't make sense graph the circle and the points and you'll see what I mean.
What is the general equation for a circle? There should be three unknowns, namely (a,b), the coordinates of the centre of the circle, and r the radius of the circle. You have three points on the circle, which should give you three equations. Solve the system, and you get a, b and r.
The second part is similar: what is the general expression for a quadratic function? Once again, there are three unknowns, use the three points to get a new system and solve.
