Question

which of the following is not an element? a water (h₂o) b gold (au) c oxygen (o₂) d copper (cu)

Explanation:

An element is a pure substance made of only one type of atom. Gold (Au), Oxygen ($O_2$), and Copper (Cu) are elements (Oxygen is a diatomic element). Water ($H_2O$) is a compound made of hydrogen and oxygen atoms, so it's not an element.

Answer:

A. Water ($H_2O$)