What physical state of material usually has lower thermal conductivity?

The physical state of materials that typically has lower thermal conductivity is gases. This is primarily due to the large spaces between gas molecules compared to liquids and solids. In gases, the molecules are far apart and move freely, which limits the ability to transfer thermal energy efficiently. Thermal conductivity relies on the interactions and collisions between molecules to transfer heat; therefore, the more tightly packed the molecules are—as in solids and liquids—the more effective they are at conducting heat.

In solids, particles are closely packed and vibrate in fixed positions, making heat transfer more efficient through conduction. Liquids have molecules that are closer together than in gases, allowing for better thermal conductivity than gases but less than solids. Plasmas consist of highly energized particles, and while they can conduct heat, the low density and spacing often lead to lower effective thermal conductivity when compared to solids and sometimes even liquids.

Thus, due to the molecular spacing and the way heat is transferred in different states of matter, gases exhibit the lowest thermal conductivity among the options provided.