The correct answer is a) solid.
High density in the context of states of matter refers to the mass of a substance per unit volume. Solids generally have particles that are closely packed together, resulting in a higher density compared to liquids and gases. For instance, metals like iron or lead have a much greater density than water, which is a liquid, or air, which is a gas. While liquids can also have a significant density, solids typically surpass them. Therefore, among the given options, solids are the state of matter most commonly associated with high density.