Which term defines a substance that flows and conforms to the outline of its container?

The correct answer is "Fluid" because this term encompasses both liquids and gases, which are substances that flow and take the shape of their container. Fluids are characterized by their ability to deform and move easily, adapting to the confines of their surroundings. In contrast, solids maintain a fixed shape and do not conform to the shape of their container; they resist deformation. Gases, while they do flow, are specifically a state of matter that expands to fill the volume of their container but, by definition, are still classified under the broader category of fluids. "Rock" refers to a solid material and does not exhibit the flowing characteristics associated with fluids. Thus, fluid is the most accurate term to describe substances that can flow and conform to the outlines of their container.