Which nonmetal exists in rhombic, monoclinic, and plastic forms depending on the temperature?

Sulfur is the nonmetal that exists in rhombic, monoclinic, and plastic forms depending on the temperature. These different forms are referred to as allotropes, which are different structural forms of the same element.

At room temperature, sulfur typically exists as rhombic sulfur (the most stable form), which consists of crown-shaped molecules. As the temperature increases, sulfur can transition to its monoclinic form, which is another stable configuration at higher temperatures. When sulfur is heated further to around 100°C, it can enter a "plastic" phase, where it behaves more like a viscous material.

This ability to exist in various allotropes at different temperatures is a unique characteristic of sulfur that demonstrates the dynamic nature of its molecular structure and how thermal energy impacts its physical properties. Other nonmetals mentioned do not exhibit such a range of allotropes under varying temperatures, making sulfur distinct in this aspect.