Which monomer joins different polymer chains together?

The correct answer is cross linker, as this term specifically refers to a type of monomer that is utilized to chemically bond or connect different polymer chains, thereby creating a more complex three-dimensional structure. Cross linking is a critical process in various applications, including the formation of gels and elastomers, where enhanced strength, stability, and durability are desired.

In contrast, the other options have distinct functions that do not pertain directly to joining polymer chains. Adhesives are substances used to bind surfaces together but do not inherently create chemical cross links between polymers. Photoinitiators are compounds that facilitate the curing process in certain types of polymers by absorbing light and initiating a chemical reaction, but they do not link chains together themselves. Oligomers are short chains of monomers, which can be components of a polymer but do not serve the specific role of cross-linking. Understanding these distinctions helps clarify why cross linker is the appropriate choice in the context of polymer chemistry.