______ Has a Hf of zero?

ΔHf of an element in its standard state is zero.

Br⁻ and Na⁺ are ions, while H₂O and CO₂ are compounds. All of them are not elements in its standard state. Thus, none of them has a ΔHf of zero.

...... The answer is : E. None of the above

None of the above. None of these are element in their standard states.