we know that
the general equation of a circle is
(x-h)²+(y-k)²=r²
where
(h,k)----> is the center of a circle
r----------> is the radius
we have
x²+y²=4------------> x²+y²=2²-------> (x-0)²+(y-0)²=2²
then
the center (h,k) is -------------> (0,0)
and radius r=2 units
the answer is the option A.(0, 0)