The smallest particle of a compound that retains the properties of the compound is called a(n)
a. atom.
b. matter.
c. element.
d. molecule.

The smallest particle of a compound is a molecule.  Even though molecules are composed of atoms, atoms only exhibit the properties of the individual element they are associated with, molecules combines the properties of all of the atoms to make the properties of the compound.  
You can kind of see this by the fact that compounds that have the same molecular formulas but different structures can have completely different properties since it shows that the molecule determines the properties of the compound.
I hope this helps.