____ are extremely long chains of molecules, with each chain containing millions of molecules.

Polymers are extremely long chains of molecules, which are formed by the repeated linking of smaller units known as monomers. Each polymer chain can consist of millions of these monomer units, leading to a vast array of substances with diverse properties. This characteristic makes polymers integral in many fields, such as materials science and biology, as they can be engineered for specific functions.

Monomers are the building blocks but do not represent the long chains themselves. Sugars are a type of carbohydrate and can be monomeric or polymeric, but they do not embody the concept of extremely long chains as a general category. Solvents are substances that dissolve solutes to form a solution, and they do not pertain to structured chains of molecules like polymers do. Thus, the defining quality of polymers being long chains of numerous molecular units precisely identifies them as the correct answer in this context.