Which gas is used in the preparation of soda water? 

Soda water is carbonated water where carbon dioxide is infused into water user pressure through a process called carbonation.

Soda water has a chemical formula ${\mathrm{H}}_2{\mathrm{CO}}_3$. Thus, soda water is also a dilute solution of carbonic acid. It is also known as sparkling water or seltzer water.

The carbonation effect is often observed in cold drinks thus these drinks are often called carbonated drinks. Cold drinks have dissolved carbon dioxide in them, and this gas is responsible for the phenomenon of effervescence which creates fizz in the bottles the moment you open them.

Hence, the correct option is D, Carbon dioxide.