A suspension of fine particles of liquid or solid in gas is defined as:

The correct answer is defined as an aerosol, which refers to a mixture where fine particles of liquid or solid are dispersed in a gas. This definition accurately encapsulates the characteristics of aerosols, which can include various substances such as pollutants, medications in inhalers, and even natural phenomena like mist or fog.

Aerosols are notable for their ability to remain suspended in the air for extended periods, which is significant in fields such as medicine, especially in the delivery of therapeutic agents through inhalation. The term can also describe a wide range of applications, from environmental science (where aerosols can affect climate and air quality) to industrial applications (such as sprays).

Moist air refers to air that contains water vapor but does not imply the presence of fine particles within it. Humidity specifically measures the amount of water vapor present in the air and does not relate to the suspension of other particles. Steam is the gas phase of water but, again, is not a term that covers the wider concept of various fine solid or liquid particles suspended in a gas. Thus, aerosol is the most appropriate term to define the concept of a suspension of particles in gas.