What is the primary function of eukaryotic RNA polymerase II?

The primary function of eukaryotic RNA polymerase II is to encode all protein-coding genes. RNA polymerase II is responsible for transcribing messenger RNA (mRNA) from DNA, which serves as a template for synthesizing proteins. This enzyme binds to promoter regions of genes and facilitates the formation of an RNA strand that corresponds to the DNA sequence of the gene being expressed.

While other options involve different RNA types and processes, they do not represent the main role of RNA polymerase II. For instance, tRNA and rRNA are synthesized by different RNA polymerases; tRNA is typically transcribed by RNA polymerase III, and rRNA is synthesized by RNA polymerase I and RNA polymerase III, depending on the type of rRNA. The production of small interfering RNA (siRNA) and microRNA (miRNA) also involves distinct pathways and enzymes, often involving Dicer and other proteins, rather than RNA polymerase II directly handling their transcription. Thus, the correct answer highlights the central role of RNA polymerase II in gene expression and the synthesis of proteins via mRNA.