Option 1: Lack of color - Gases can be colorless or have color. Therefore, this option does not accurately distinguish gases from other forms of matter.
Option 2: Ability to flow - Gases have the ability to flow and take the shape of their container. This is one of the distinctive characteristics of gases and can help in distinguishing them from solids or liquids.
Option 3: Inability to form free surfaces - Gases do not have a definite volume or shape and can freely diffuse and mix with each other. They lack the ability to form free surfaces or maintain their shape. This characteristic sets gases apart from liquids and solids.
Option 4: Ability to exert a buoyant force - While gases can exert a buoyant force, this property is not unique to gases and can also be observed in liquids. Therefore, this option does not accurately distinguish gases from other forms of matter.
Therefore, the correct answer is option 3 - the inability to form free surfaces. This characteristic is one of the key distinctions of gases from other forms of matter, along with their ability to flow.