Final answer:
A fluid is a substance that can move freely and adapt to the shape of its container, and this includes liquids, gases, and plasmas. These states do not have a definite shape and have molecules that can move around freely, unlike solids.
Step-by-step explanation:
The substance characterized by its ability to move freely and adapt to the shape of the vessel that contains it is known as a fluid. These substances can be either a liquid or a gas as both take the shape of their containers and have molecules that can move around freely. When we discuss the phases of matter where molecules are capable of changing their positions, the correct answer would be c) liquid, gas, plasma. Solids have a definite shape and volume, thus their molecules are fixed in place and cannot move freely. In contrast, liquids and gases do not have a definite shape. Plasmas, like gases, do not have a definite shape nor volume, allowing their molecules to move freely as well.
In summary, the phases of matter that exhibit fluidity are liquids, gases, and plasmas because of their ability to yield to shearing forces, while solids resist them. Therefore, to answer the question, substances capable of moving freely and adapting to their container’s shape can be described as fluids, specifically in the states of liquid, gas, and plasma.