Balloons are filled with helium.

Helium is the correct answer for the gas typically used to fill balloons. Let's explore why helium is the preferred choice for filling balloons:

1. Lightness and buoyancy:
- Helium is a lighter-than-air gas, which means it has low density compared to the surrounding air.
- When a balloon is filled with helium, the gas inside is less dense than the air outside, causing the balloon to float or rise.
- This property of helium makes it ideal for creating floating balloons, as it allows them to stay aloft in the atmosphere.

2. Availability and safety:
- Helium is abundantly available on Earth, although it is a finite resource.
- It can be extracted from natural gas deposits, making it a relatively accessible gas for various applications.
- Helium is also considered safe to use in balloons. It is non-toxic, non-flammable, and does not react with other substances, making it a suitable choice for filling balloons that are intended for general use.

3. Inertness:
- Helium is an inert gas, which means it does not readily react with other elements or compounds.
- This inertness ensures that the gas inside the balloon remains stable and does not interact with the balloon material or its surroundings.
- This property is particularly important for the longevity and safety of the balloons, as it reduces the risk of the balloon bursting or causing any chemical reactions.

4. Other gases:
- While helium is the most commonly used gas for filling balloons, other gases such as hydrogen, nitrogen, oxygen, and argon can also be used.
- Hydrogen is also lighter than air and can provide buoyancy similar to helium. However, it is highly flammable and poses safety risks, so it is not commonly used for filling balloons.
- Nitrogen and oxygen are denser than air and do not provide the necessary buoyancy for floating balloons.
- Argon, like helium, is an inert gas, but it is heavier than air, making it unsuitable for creating floating balloons.

In conclusion, helium is the preferred gas for filling balloons due to its lightness, availability, safety, inertness, and ability to create floating balloons.