Answer: molecule/ compound
Explanation: The given analogy is
Link : chain :: atom : _
As we can see that link is the term which can be used to form a chain. It is a connecting term for link.
Similarly atoms is the term which can be used to define the molecule or the compound. As atom is the tiniest part of the molecule or compound just like the link is the smallest part of the chain.
The best fit analogy out of the give options is molecule/ compound.