Which of the following gases has the highest density under standard conditions?

As we know density is directly proportional to molecular mass.

So, when molar mass increases, density also increases and vice versa.

Now we have to see which gas among these has the highest molar mass.

Molar mass of ${SO}_2=64 g/mol$ is highest among all.

Hence, the density of ${SO}_2$ under standard conditions is also highest. 

Therefore, option (D) is correct.