A large molecule composed of many repeating subunits called monomers.

Here's why this is the best explanation:

* Monomers: The building blocks of polymers are small molecules called monomers.

* Repeating Subunits: Polymers are formed by linking many monomers together in a chain-like structure. This repeating pattern is what defines the polymer.

* Large Molecule: The result of this linking process is a very large molecule, often with a high molecular weight.

Let's look at why the other options are not as accurate:

* A molecule that can be broken down into smaller molecules: While polymers can be broken down, this is a consequence of their structure, not their definition.

* A molecule made up of many different types of atoms: While polymers often contain many different atoms, the defining characteristic is the repeating monomer unit.

* A molecule that is very strong and durable: While some polymers are strong and durable, this is a property of the specific polymer, not a general definition.

In summary, the core concept of a polymer is that it is a large molecule built from many repeating units (monomers).