Which of the following RNAs is not required for the synthesis of protein?

To determine which RNA is not required for the synthesis of proteins, we first recall the roles of the main types of RNA involved in translation.

1. Messenger RNA (mRNA) – carries the genetic code from DNA to the ribosome, where it is read to assemble amino acids into a protein. Because it provides the template for protein synthesis, mRNA is essential.

2. Transfer RNA (tRNA) – matches each codon on the mRNA with the correct amino acid by delivering the amino acid to the ribosome. Without tRNA, the ribosome could not read the codons, so tRNA is also essential.

3. Ribosomal RNA (rRNA) – forms the core structural and catalytic components of ribosomes, the molecular machines that carry out translation. rRNA is therefore required for protein synthesis.

4. Small interfering RNA (siRNA) – is involved in the RNA interference (RNAi) pathway, where it guides the degradation or translational repression of specific mRNA molecules. Its purpose is regulatory, not to provide the template, adaptors, or ribosomal machinery needed for building proteins.

Since siRNA does not participate in the translation process itself, it is the RNA that is not required for protein synthesis.

Hence, the correct answer is siRNA.