Answer: C - Subunit (Monomer) is always a molecule
Step-by-step explanation: To narrow this down, b, cannot be the answer because it is known that Polymers can extend forever with enough materials present.
It cannot be d) or a) because atoms need to have full bonds, that is, a subunit must be bonded to other units to even be considered a subunit. So it must be c) because a subunit must have full bonds. If you are talking about organic polymers, at the very least, subunits must be bonded with hydrogens, subunits cannot just be carbon atoms, same thing goes with inorganic subunits
Hope this helped and good luck with Chem!