What characteristic defines polar solvents?

Polar solvents are defined by their ability to dissolve in water due to their molecular structure, which promotes interactions with water molecules. These solvents typically have a significant difference in electronegativity between atoms, creating a dipole moment, which leads to their polarity. This characteristic allows them to mix with and dissolve ionic or other polar compounds effectively.

In the options provided, the description of polar solvents as "water soluble flammable liquids" accurately reflects their capability to dissolve in water while often being flammable. This solubility is crucial in various applications, including cleaning, chemical reactions, and industrial processes.

Other choices do not accurately capture the essence of polar solvents. For example, flammable liquids that resist water would imply a non-polar nature, while non-combustible materials do not align with the idea of solvents, which generally include flammable substances. Lastly, gases that ignite easily lack the structural attributes needed to be classified as polar solvents. Because of these properties, the answer focuses on the correct identification of polar solvents as water-soluble and potentially flammable.