Select the gas.

 - water in a sink
 - door
 - tortoise shell
 - helium
A door is a solid. A solid has a size and shape of its own. When you open or close a door, it keeps its shape.
The water in a sink is a liquid. A liquid takes the shape of any container it is in. If you move the water from a sink into a different container, the water will take the shape of that container. But the water will still take up the same amount of space.
Helium is a gas. A gas expands to fill a space. Helium is lighter than air. So, if you fill a balloon with helium, the balloon will rise. If helium leaks out of the balloon, the helium will expand into the space around the balloon.
A tortoise shell is a solid. A solid has a size and shape of its own. A tortoise shell is made of a solid called keratin, just like your fingernails!