Chemistry - Thermochemistry Question with Solution | TestHub

${\Delta }_f{H}^{\circ }$ is zero only for the most stable form of an element in its standard state.

${\mathrm{I}}_2$ is a solid at 298 K, so ${\mathrm{I}}_2(\mathrm{s})$ qualifies.

${\mathrm{Br}}_2$ is a liquid (not gas), graphite (not diamond) is the standard form of carbon, and ${\mathrm{O}}_2$ (not ${\mathrm{O}}_3$) is the standard form of oxygen.

Answer: (C).