Which of the following non-metal is lustrous? 

Iodine

The correct answer is option 4. Iodine.

Iodine is a non-metal element that exhibits some unusual properties compared to other non-metals. In its solid form, iodine appears as dark, shiny, metallic-looking crystals. This appearance is due to the arrangement of iodine atoms in a crystalline lattice structure.

The lustrous or shiny appearance of solid iodine is a result of its unique molecular structure and the way it interacts with light. The iodine molecules are arranged in layers within the crystal lattice, allowing them to reflect and refract light in a way that gives the surface a metallic-like shine.

While iodine is classified as a non-metal due to its position in the periodic table and its chemical properties, its solid form can exhibit properties typically associated with metals, such as luster. This is in contrast to other non-metals like fluorine, chlorine, and bromine, which typically do not have a shiny appearance in their solid states.

It's important to note that despite its metallic luster, iodine is still a non-metal and possesses non-metallic properties in terms of its chemical behavior, such as being a poor conductor of electricity and lacking the malleability and ductility characteristic of metals.