Answer is: 2) a compound.
A chemical compound is a chemical substance composed of many identical molecules composed of atoms held together by chemical bonds.
A compound is a pure substance because its molecule cannot be broken down into simpler particles by physical means.
This is example of diatomic compound.
Diatomic molecules are molecules made of two atoms.
They can be homonuclear (molecule made of two atoms of the same element) and heteronuclear (molecule made of two different atoms, like in this example).