Answer & Explanation:
The law of multiple proportion states that two elements A and B can react with each other to form multiple compounds while one of the two elements remain fixed.
For Example
Hydrogen reacts with oxygen to form:
Water (H2O)
Hydrogen peroxide (H2O2)
The one you have selected is correct. CO is a compound because it contains more than one element.
Answer : The molecular formula of the compound is, 
Explanation :
Molecular formula : It is the representation of substance by the symbols and it denotes the number of atoms of each element present in the compound.
Now count the number of carbon, hydrogen, nitrogen and oxygen atoms present in the given compound.
As we see that in the given compound, there are 8 atoms of carbon element, 3 atoms of oxygen element, 1 atom of nitrogen element, 9 atoms of hydrogen element.
Thus, the molecular formula of the compound will be 
Filtration is a method for separating an insoluble solid from a liquid.
Solute of solution = 17.8 g
Solvn = 198 g
% = 17.8 / 198
w% = 0.089 x 100 = 8.9% by mass
hope this helps!