Explanation

Diamond is a non-metal with a high-density.

Graphite: A non-metal but has a lower density (~2.2 g/cm³).

Hydrogen: The lightest element with very low density (~0.000089 g/cm³).

Helium: Also very low density (~0.00018 g/cm³).

Diamond: A non-metal form of carbon with high density (~3.5 g/cm³).

Iron: A metal, so not a valid choice for a non-metal category.