Question

What is the smallest unit of a compound ?

Answer 1

*Answer:*
The question is ambiguous. The answer could be, an element, or it could be a molecule.
*Explanation:*
Compounds are built up of molecules. For example, sodium chloride consists of many molecules of sodium chloride. However, a molecule of sodium chloride consists of atoms of sodium and chlorine.

Answer 2

a compund is formed due to molecles and the are formed by elements 
thus i think the smallest unit is element