Which of the following has a standard enthalpy of formation equal to zero at $298\mathrm{K}$?

By convention the standard enthalpy of formation of an element in its most stable form at $298\mathrm{K}$ and $1\mathrm{bar}$ is zero. Bromine is liquid at $298\mathrm{K}$; gaseous bromine, diamond (graphite is the reference) and ozone (dioxygen is the reference) are not the reference forms.