Final answer:
A water molecule is a pure substance because it has a consistent composition and distinct chemical properties, unlike ocean water, which is a homogeneous mixture.
Step-by-step explanation:
Among the items listed - ocean water, a water molecule, hydrogen atom, skin cell, liver, elephant, wolf pack, tropical rainforest, planet Earth - the one that represents a pure substance is a water molecule. A pure substance has a consistent composition and distinct chemical properties. In contrast, ocean water is a homogeneous mixture containing various substances such as salt and water.
Therefore, when identifying a pure substance, one must look for materials that are not mixed with other substances and have a uniform and definite composition. A water molecule, which consists of two hydrogen atoms and one oxygen atom bonded together, is a perfect example of a pure substance because it fits these criteria.