What can be inferred about a substance with high density?

A substance with high density is best characterized as being compact and having significant mass per unit volume. This means that for a given volume, a dense material contains a greater amount of mass compared to less dense materials. For example, lead has a higher density than aluminum, which means that lead has more mass in the same volume compared to aluminum.

High density does not necessarily imply that a substance is lightweight or easily floatable, as indicated in the first option. In contrast, substances with high density often sink in fluids rather than float. While it's possible for dense materials to exist in various states (solids, liquids, or gases), high density does not restrict a substance to being always a solid, dismissing the third option. Finally, buoyancy relates to the ability of an object to float in a fluid, and while a denser object has less buoyancy than a less dense one when placed in a fluid, this does not represent the general definition or implications of high density. Thus, the most accurate inference regarding density is that it signifies a compact arrangement of mass within a given volume, which aligns with the correct answer.