Noble gases exist as single, non-bonded atoms (monatomic) with a fully filled outer electron shell, so the only force holding the atoms together is very weak London dispersion (van der Waals) forces.

Because these weak intermolecular forces are easily overcome, noble gases have very low boiling and melting points and remain gaseous at room temperature.

Halogens, by contrast, exist as diatomic molecules whose intermolecular forces strengthen with increasing molecular size down the group, so fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.

Noble gases are also chemically inert, so options citing strong bonds or high reactivity are incorrect, and atomic size alone does not explain the physical state.

Hence, noble gases remain gases due to the weak intermolecular forces between their atoms.