How is a gas defined according to the provided information?

A gas is defined as a substance that has a vapor pressure greater than 300 kPa at 50 degrees Celsius. This definition emphasizes the physical properties that distinguish gases from liquids and solids, primarily the capability of gases to exert pressure in their gaseous state due to their high kinetic energy. At higher temperatures, such as 50 degrees Celsius, the increased kinetic energy allows the substance to transition from a liquid state, if applicable, into a gas, leading to a higher vapor pressure. This characteristic is a fundamental aspect of gases, highlighting their behavior under specific temperature and pressure conditions.

In contrast, other options focus on qualities that do not accurately encapsulate the concept of a gas. For example, a substance being flammable at standard pressure does not inherently define it as a gas, as many liquids and solids can also be flammable. Similarly, a low boiling point is a trait that applies to many liquids and is not exclusive to gases. Finally, stating that a substance is solid at room temperature outright disqualifies it from being classified as a gas, as gases are defined by their state and behavior under varying temperature and pressure conditions.