Question

What is the equation of a circle centered at the origin with radius 20?

Answer 1

We know that
the equation of a circle is
(x-h)²+(y-k)²=r²
where 
(h,k)-----> is the center
r--------> is the radius

is the center is the point (0,0) and radius  r=20
then
(h,k) is (0,0)

(x-0)²+(y-0)²=20²---------> (x)²+(y)²=400


the answer is
the option D.)  x2 + y2 = 400