Which of the following describes a gas at standard atmospheric conditions?

A gas at standard atmospheric conditions is characterized by its ability to expand to fill the entire volume of its container. This property arises from the kinetic molecular theory, which states that gas molecules are in constant, random motion and have a significant amount of space between them. Because of this movement and spacing, gases do not have a fixed volume; instead, they will spread out and occupy the available space uniformly.

In contrast, the other options describe characteristics that do not apply to gases. A gas does not occupy a definite volume, as it will adapt to the size of its container. It is also not incompressible — gases can be compressed due to their relatively large intermolecular spaces, unlike liquids and solids, which exhibit incompressibility. Furthermore, gases do not have a fixed shape; they take the shape of whatever container they are placed in, which is contrary to the definition of solids or liquids that retain specific shapes.

Thus, the correct description of a gas at standard atmospheric conditions is that it expands to fill its container.