What are the building blocks for plastics?

Plastics are primarily made from polymers, which are long chains of repeating molecular units known as monomers. The properties of plastics, such as flexibility, strength, and durability, are derived from the structure and arrangement of these polymer chains. Polymers can be synthesized through processes like addition polymerization or condensation polymerization, allowing for the creation of a wide variety of materials with different characteristics.

In terms of the other options, composites are materials made from two or more constituent materials with significantly different physical or chemical properties, and while they may incorporate plastics, they are not the fundamental building blocks of plastics themselves. Filaments are typically strands of material, often used in 3D printing, and solvents are substances that dissolve solutes to form a solution but do not serve as the fundamental components for creating plastics. Therefore, polymers are indeed the correct answer as they form the structural basis upon which various kinds of plastic materials are constructed.