Question 3
By chemical composition, pure substances are divided into which two kinds?
The right answer is the one marked above.
Elements and compounds. An element holds one kind of atom only — iron (Fe), oxygen (O2). A compound holds different elements chemically joined — water (H2O), carbon dioxide (CO2).
Homogeneous and heterogeneous is the division one branch further along — it splits mixtures, not pure substances. Solid, liquid and gas is the division by physical nature, which is a different question about the same matter.