What is the smallest particle of a compound that retains the characteristics of the compound?

The smallest particle of a compound that retains the characteristics of that compound is indeed a molecule. A molecule is formed when two or more atoms chemically bond together, and it embodies the specific properties of the compound it represents. For example, water (H₂O) is a molecule consisting of two hydrogen atoms and one oxygen atom. This molecule exhibits the unique properties of water, such as its liquid state at room temperature and its polarity, which affect its behavior in different contexts.

In contrast, an atom is the basic unit of matter that cannot be broken down into simpler substances while retaining its chemical properties, but it does not represent a compound on its own. An ion refers to an atom or molecule that carries a charge due to the loss or gain of electrons, but ions are not necessarily representative of a compound's structure or properties in their ionic form. Electrons are subatomic particles with a negative charge that do not retain the characteristics of any compound, as they are constituents of atoms rather than complete units of compound matter. Thus, molecules are the correct answer because they are the smallest entities that embody the properties and characteristics of chemical compounds.