Which graph represents a circle with equation x2 + y2 = 9

The basic form of the equation of a circle is: 
x^2+y^2=r^2 
where r^2 is the radius squared.

Looking at your equation, x^2+y^2=9, the radius is 3 (sqrt of 9 is 3) 

Therefore, the graph B would be correct since the radius of the circle at all points is 3. 

Hope I helped :)