The correct answer is: b a solid forming a vapour on heating and reforming as a solid on cooling.
Explanation: Sublimation is a phase transition phenomenon where a solid changes directly into a gas without passing through the liquid state. This process occurs when the particles within the solid gain enough energy, typically through heat, to break free from their rigid structure and disperse into the atmosphere as vapor. When the temperature drops, these gaseous particles can lose energy and revert back to the solid state, typically in the form of frost or snow. Examples of sublimation include dry ice turning into carbon dioxide gas and the transition of snow directly into water vapor. Options a and c describe different physical processes that are not representative of sublimation.