The functional RNA that is a primary component of the structures required for protein synthesis is ribosomal RNA (rRNA).

Here's why:

* Ribosomes are the protein synthesis machinery: Ribosomes are complex structures made of proteins and rRNA. They are the sites where mRNA is translated into protein.

* rRNA is the structural backbone of ribosomes: rRNA provides the framework for the ribosome's structure. It also plays a crucial role in the decoding of mRNA and the formation of peptide bonds during translation.

While other functional RNAs like tRNA and mRNA are involved in protein synthesis, rRNA is the essential structural component that makes the process possible.