State the correct statement for the following compounds by calculating the ratio by mass of the combining elements.

Ammonia has a ratio by mass of $14:3$. One nitrogen atom and three hydrogen atoms make up the compound Ammonia ($N{H}_3$) as ($14:1\times 3$). 

Hence the mass ratio is $14:3$.

Aluminium fluoride has a ratio by mass of $9:19$. One aluminium atom and three fluorine atoms make up the compound Aluminium fluoride ($\mathit{Al}{F}_3$) as ($27:19\times 3$). As a result, the mass ratio is $9:19$.

Magnesium sulphide ($\mathit{MgS}$) has a ratio of $3:4$. The atomic mass of magnesium is 24, and sulphur is 32 ($24:32$). Hence the ratio is $3:4$.

The atomic mass of carbon is 12 and oxygen is 16 in carbon monoxide ($\mathit{CO}$) as ($12:16$). Therefore, the ratio of mass is $3:4$. Hence, the correct statement is Carbon monoxide has a ratio by mass of $3:4$.