Which substance is formed by combining many small molecules into long chain-like structures?

The correct choice is polymer because a polymer is a complex structure formed when small molecules, known as monomers, chemically bond together in long chain-like formations. This process is called polymerization. Polymers can take on various physical and chemical properties depending on the types of monomers used and the nature of their bonds. They are fundamental in various applications, including plastics, rubber, and even biological structures like proteins and DNA.

In contrast, a composite refers to a material made from two or more constituent materials with different physical or chemical properties, which differs from the single-chain structure characteristic of polymers. A gel usually has a semi-solid, jelly-like consistency, often resulting from a polymer network but is not defined by the long chain structure itself. A liquid is a state of matter characterized by free-flowing molecules; it does not have the long chain structure inherent in polymers. Thus, the definition and nature of polymers precisely align with the question about substances formed from many small molecules into long chain-like structures.