Which of the following structures assists in protein synthesis?

Ribosomes are the primary structures responsible for protein synthesis in both prokaryotic and eukaryotic cells. They serve as the site where messenger RNA (mRNA) is translated into polypeptide chains, which then fold into functional proteins. Ribosomes can either be free-floating in the cytoplasm or attached to the endoplasmic reticulum, significantly contributing to the overall process of gene expression and the translation of genetic information into functional proteins.

While mitochondria and chloroplasts also have roles related to protein synthesis, especially in their own organelle-specific genes, it is ribosomes that directly translate mRNA into proteins across all types of cells. The nucleus is involved in the synthesis of mRNA (through transcription), but it is not directly involved in the synthesis of proteins.