Which of these substances has a definite shape and definite volume?

The substance that has a definite shape and definite volume is a solid. In solids, the particles are tightly packed together in a fixed arrangement, which gives them a specific shape that does not change unless an external force is applied. This structure allows solids to maintain their shape and volume under normal conditions, making them distinct from liquids and gases.

In contrast, liquids have a definite volume but take the shape of their container, meaning they do not have a fixed shape. Gases, on the other hand, do not have either a definite shape or volume, instead expanding to fill the entire space available to them. Aerogels, while having unique properties and potentially being lightweight, are also classified as solids; however, they do not exhibit the same rigid characteristics typically expected in more conventional solids. Therefore, the defining features of solids make them the correct answer for this question.