Which substances have ah = 0 kj/mol

Oxygen, bromine, iron, helium

Explanation:

is defined as the standard enthalpy of formation. By definition, the standard enthalpy of formation is equal to 0 kJ/mol for the substances in their standard states, that is, at room temperature and 1 atm pressure.

Simply speaking, looking at the substances given, we need to understand whether their states agree with what we expect to see at standard conditions (e. g., sodium is a metal, fluorine is a gas, bromine is a liquid at standard conditions). Those are substances consisting of just one type of atoms.

catya that's not even a b or c