Argon is a commonly used gas to fill incandescent light bulbs because argon is . . . . . . . . A. lightest B. reactive C. abundant D. inert

The correct answer is: D. inert.

Argon is a noble gas, which means that it is very stable and does not react with other elements. This makes it an ideal gas to fill incandescent light bulbs, as it will not react with the filament and cause it to break down.

The other options are incorrect for the following reasons:

• A. Argon is not the lightest gas. Hydrogen is the lightest gas.
• B. Argon is not reactive. It is a noble gas, which means that it is very stable and does not react with other elements.
• C. Argon is not abundant. It is the third most abundant gas in the Earth’s atmosphere, but it is not as abundant as nitrogen or oxygen.