Explanation

Hydrogen gas has a thermal conductivity of about 0.18 W/m-K, whereas liquid water is about 0.60, aluminium about 237, and copper about 401 W/m-K. Gases conduct heat far more poorly than liquids, and enormously more poorly than metals. Hydrogen therefore has the least coefficient of thermal conductivity among the four. Note: UPSC's published key marks option A, but the standard conductivity data clearly place hydrogen lowest.