The correct answer is A. Air
Step-by-step explanation:
A fluid refers to a substance that flows or deforms due to external factors and other types of factors, in general terms, fluids include all liquids, gases, and substances in plasma state as in all of these molecules do not have a fixed arrangement but can move and therefore the substance can flow. On the other hand, solids are not fluids because due to the way molecules are organized and bonded in them they do not commonly deform or flow. This implies, the fluid substance is air, because this is a gas and thus a fluid, while others such as ice, sand, and candles are solids.