Final answer:
Iron (D) is an element, as it is a pure substance consisting of one type of atom and cannot be broken down by chemical means. Other options are mixtures and compounds, not elements.
Step-by-step explanation:
Among the options provided, iron (D) is an example of an element. An element is a pure chemical substance consisting of one type of atom, which cannot be broken down into simpler substances by ordinary chemical means. Elements such as hydrogen, oxygen, carbon, nitrogen, sodium, chlorine, and iron are fundamental substances from which all matter is made. Other options like concrete, salt, and steel represent a heterogeneous mixture, a compound containing 2 elements, and a metal alloy, respectively. Iron, represented by the chemical symbol Fe on the periodic table, is known for being shiny, malleable, ductile, and a good conductor of electricity and heat, which are typical properties of metals.