Final answer:
Argon (Ar) is the gas most commonly used in thermal windows for insulation because of its inertness, non-toxicity, and low thermal conductivity.
Step-by-step explanation:
The gas that is often used in thermal windows between the panes is Argon (Ar). Argon is an inert gas, meaning it does not react with other substances, and it is not harmful to humans or the environment. It is chosen for these types of applications because it has low thermal conductivity, which makes it excellent for insulation and thus improves the energy-efficiency of the windows.
Other gases like krypton or xenon can also be used, but argon is more common due to its cost-effectiveness and availability. The options provided are Cl (chlorine), Ar (argon), N (nitrogen), and O (oxygen), out of which Ar is the correct answer.