Question

what is the smallest particle representing water?
a hydrogen atom
an oxygen atom
a molecule
diatomic oxygen

Explanation:

Water is composed of hydrogen and oxygen atoms. A water molecule ($H_2O$) is the smallest particle that has the chemical properties of water. An individual hydrogen or oxygen atom does not represent water, and diatomic oxygen ($O_2$) is not related to the basic - particle of water.

Answer:

a molecule