Circle perimeter/circumference = 2*Pi*Radius (or Pi*Diamter)
Square perimeter = 4 * Side length

if our hypothetical square and circle are equal in perimeter than the side of a square equals the diameter of the circle. Let's pretend S and D both equal 10
Square area = 10^2 = 100
Circle area = Pi * 5^2 = 78.5
***5 comes from the radius of the circle being half of the diameter, 10/2 = 5

The ratio is roughly 3/4 which is answer choice C