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.**