Final answer:
Helium is an element, which is a pure substance consisting of only one type of atom, and thus cannot be a compound or a mixture.
Step-by-step explanation:
Helium is a pure substance that can be classified as an element. It is not a compound, as it contains only one type of atom, which cannot be broken down into simpler substances through chemical means. Helium is not a mixture because it is not a combination of different substances. Homogeneous and heterogeneous refer to types of mixtures, therefore they do not apply to helium. Furthermore, helium is definitely not an electron, which is a subatomic particle found in all atoms.