Which statement correctly describes a supercritical fluid?

A supercritical fluid is characterized by exhibiting properties of both gases and liquids, which is why the second statement is the correct choice. In a supercritical state, the substance is above its critical temperature and critical pressure, which allows it to bypass the distinct phase boundaries found in typical gases and liquids. This leads to unique properties, such as a density similar to that of a liquid while still having the ability to diffuse like a gas.

For instance, supercritical CO2 is used in various applications, such as extraction processes, because it can dissolve substances like a liquid while also having lower viscosity akin to a gas.

The other statements do not accurately capture the behavior of supercritical fluids. Being below critical pressure does not describe a supercritical fluid, as such fluids are found above this threshold. Supercritical fluids are generally not associated with low temperatures but rather exist at high temperatures and pressures. Finally, while some specific supercritical fluids might be more common for certain chemicals, the concept itself is not exclusive to a limited set of substances; many different compounds can form supercritical fluids under the right conditions.