Which of the following is a poor conductor of heat ?
 

Thermal conductivity of air is very-very low 0.024.

Air is a poor conductor of heat compared to the other options. Heat conduction occurs when energy transfers between particles in a material. In metals like copper, the particles are tightly packed, allowing heat to move quickly, making them excellent heat conductors. 

Concrete, while not as good as metals, still conducts heat better than gases due to its solid structure. 

Mercury, being a liquid metal, also conducts heat, although not as efficiently as solid metals. Air, however, is a gas with particles spread far apart, which makes it very inefficient at transferring heat. This is why air is considered a poor conductor and is often used as an insulating material.