ANS:B - decreases

With an increase in porosity, the thermal conductivity of a solid substance typically decreases. Explanation: Porosity refers to the fraction of void space in a material, which can be filled with air or other gases. When a material has a higher porosity, it contains more voids or empty spaces, which act as insulating barriers to heat transfer. These voids disrupt the pathways for thermal energy to flow through the material, leading to a decrease in thermal conductivity. So, the correct answer is: decreases