The correct answer is a) solid.
Solids are the state of matter characterized by their definite shape and volume. This means that solids hold their form and do not conform to the shape of their container. The particles in a solid are closely packed together, which keeps them in fixed positions and contributes to their structure. In contrast, liquids have a definite volume but take the shape of their container, while gases neither have a definite shape nor a definite volume, expanding to fill whatever space they occupy. Plasma, another state of matter, consists of ionized gases and does not have a definite shape or volume either. Therefore, when we think of matter that maintains both a definitive shape and volume, solids are the clear choice.