Answer:
C.O
Step-by-step explanation:
"C. O" (Oxygen) is an element.
An element is a pure substance that cannot be broken down into simpler substances by chemical means. It is made up of only one type of atom, with the same number of protons in its nucleus.
A. MgBr (Magnesium bromide) is a compound, which is made up of two or more elements chemically combined in a fixed ratio.
B. HCI (Hydrogen chloride) is a compound, made up of hydrogen and chlorine.
D. Cu(SO4)3 (Copper sulfate) is a compound, made up of copper, sulfur, and oxygen.