Answer:
a
Step-by-step explanation:
The standard form of the equation of a circle is
(x - h)² + (y - k)² = r²
where (h, k) are the coordinates of the centre and r is the radius
Here (h, k) = (0, 0), thus
(x - 0)² + (y - 0)² = r², that is
x² + y² = r² → a
