Which of the following is not an example of element?

Question: Which of the following is not an example of element?

(1) oxygen(2) gold (3)water (4)hydrogen answer: water ,an element is a pure substance consisting only of atom that all have the same number of protons in their nuclei, chemical elements cannot be broken down into simpler substance by any chemical reactions ,the number of proton in the nucleus as the  defining property of an element and is referred to its atomic number.