The inert gas used as beacon light is A. Kr B. Ar C. He D. Ne

The correct answer is $\boxed{\text{Argon (Ar)}}$.

Argon is an inert gas that is used in a variety of applications, including beacon lights. It is a colorless, odorless, and tasteless gas that is non-flammable and non-toxic. Argon is also a good conductor of electricity, which makes it ideal for use in beacon lights.

The other options are incorrect because they are not inert gases. Helium (He) is a noble gas that is lighter than air. Neon (Ne) is also a noble gas that is used in neon signs. Krypton (Kr) is a noble gas that is used in fluorescent lamps.