What is the Density of CO gas if 0.196g occupies a volume of 100mL?

Density
= Mass / Volume
= (0.196 g) / (100 mL)
= 0.00196 g/mL

(The answer can also expressed as 0.00196 g/cm³ or 1.96 g/L.)

1.96 grams/ liter (There are 1000 ml in a liter so multiply 100 ml volume times 10 to get the volume per liter.)