Which of the following nonmetals does NOT have multiple allotropes at room temperature?

Hydrogen is the nonmetal that does not have multiple allotropes at room temperature. Allotropes are different forms of the same element in the same physical state, which can exhibit different physical and chemical properties.

For hydrogen, there is primarily one diatomic form (H₂), which is the molecule consisting of two hydrogen atoms. Unlike carbon, which has multiple allotropes such as graphite, diamond, and fullerenes, or oxygen, which can exist as O₂ (dioxygen) and O₃ (ozone), hydrogen does not exhibit such variability in its allotropic forms.

While nitrogen has different allotropes (like N₂ and the less stable N₃), hydrogen remains in its single stable diatomic form at room temperature under normal conditions. This simplicity in its structure and lack of alternative stable forms at this temperature is what distinguishes hydrogen from the other nonmetals listed.