Final answer:
Bromine is the halogen that is a liquid at room temperature, while fluorine and chlorine are gases and iodine is a solid, due to the differences in intermolecular dispersion forces.
Step-by-step explanation:
The halogen that is a liquid at room temperature is C. Bromine. At room temperature, halogens exhibit different states of matter primarily due to the differences in their intermolecular forces. Elements like fluorine and chlorine exist as gases, bromine is a liquid, and iodine turns into a solid. These differences are attributable to the increasing number of electrons as you move down the group, which in turn increases the strength of dispersion forces. Iodine has the highest number of electrons among them, which results in solid-state due to strong forces holding the molecules together. Bromine, with fewer electrons than iodine, has weaker intermolecular forces, allowing it to be a liquid. Conversely, fluorine and chlorine have even fewer electrons, resulting in weaker forces and a gaseous state at room temperature.
The halogen that is a liquid at room temperature is bromine. Fluorine and chlorine are gases at room temperature, while iodine is a solid.