Area of the circular sheet = $\pi$ * (20)2 = 400 $\pi$
Area of 4 smaller circles cut out = 4 * $\pi$ * (5)2 = 100 $\pi$
So, area of remaining paper = 300 $\pi$
$\therefore$ Ratio of the paper left to the area of the circles = 300 $\pi$ : 100 $\pi$ = 3 : 1
C is the answer.