Which of the following non-metal exists as solid? 

Iodine 

The correct answer is option 4. Iodine.

Iodine \((I_2)\) is a non-metal element that exists as a solid at standard room temperature and pressure (around 25°C and 1 atm). It forms shiny, dark violet crystals with a metallic luster. This solid form of iodine is a result of the weak van der Waals forces that hold its molecules together in a crystalline lattice structure.

Unlike many non-metals that exist as gases or liquids at room temperature, iodine has a relatively high melting point of approximately 113.7°C (or 236.7°F) and a boiling point of approximately 184.3°C (or 363.7°F). This is due to the intermolecular forces between iodine molecules, which are stronger than those in gases or liquids like chlorine \((Cl_2)\) or bromine \((Br_2)\).

When solid iodine is heated, it sublimes directly into a purple vapor without melting. Sublimation is the process by which a substance transitions directly from a solid to a gas phase without passing through a liquid phase. This characteristic property of iodine is often demonstrated in laboratories by heating solid iodine crystals, resulting in the release of purple vapor.

In summary, iodine is the non-metal among the options provided that exists as a solid at room temperature and pressure, forming dark violet crystals with a metallic luster.