What does the amount of 3D space occupied by a body define?

The correct answer is volume, as it specifically refers to the three-dimensional space that an object occupies. Volume is a critical concept in biomechanics because it relates to how much space a body takes up, which can influence various physical properties and functions, such as buoyancy, stability, and load distribution.

In a biomechanical context, understanding volume helps in analyzing how bodies move in space and interact with their environment. For instance, the volume of a limb or an entire body can impact movement efficiency and overall functionality.

Surface area, while important in examining relationships such as heat exchange and friction, does not account for the entirety of three-dimensional space an object occupies. Mass is a measure of the quantity of matter in an object, which is different from the physical space it takes up. Density combines mass and volume, indicating how compact an object's matter is, rather than describing the space itself directly. Thus, volume is the most appropriate term for defining the amount of three-dimensional space occupied by a body.