ANS:A - Nitrogen

Among the options provided, Nitrogen typically has the minimum thermal conductivity. Explanation:

1. Thermal Conductivity: Thermal conductivity (�k) is a material property that describes the ability of a substance to conduct heat. It represents the rate at which heat is transferred through a material per unit area per unit temperature gradient.
2. Comparison:
   • Nitrogen: Nitrogen is a gas at standard temperature and pressure (STP). Gases generally have lower thermal conductivities compared to solids and liquids because the molecules are farther apart and have weaker intermolecular forces, which restricts the transfer of heat.
   • Steel: Steel is a solid material with relatively high thermal conductivity compared to gases. Metals, in general, have high thermal conductivities due to the free movement of electrons, which facilitate heat transfer.
   • Carbon Black: Carbon black is a finely divided form of carbon with relatively low thermal conductivity. Its structure, consisting of small, densely packed particles, inhibits the transfer of heat compared to bulk carbon materials.
   • Tar: Tar is a viscous, liquid material with relatively low thermal conductivity. Liquids typically have lower thermal conductivities compared to solids, and viscous materials like tar may have additional resistance to heat transfer due to their high viscosity.
3. Conclusion: Among the options provided, nitrogen, being a gas, generally has the lowest thermal conductivity. However, it's worth noting that the thermal conductivity of materials can vary depending on factors such as temperature, pressure, and composition.