Which structure is responsible for protein synthesis?

Ribosomes are the structures responsible for protein synthesis in cells. They play a crucial role in translating the genetic information encoded in messenger RNA (mRNA) into polypeptide chains, which then fold into functional proteins. Ribosomes can either be free-floating in the cytoplasm or bound to the endoplasmic reticulum, creating rough ER, where they facilitate the synthesis of proteins destined for export or for use in the cell membrane.

The other structures listed have distinct functions: the Golgi body processes and packages proteins for distribution, mitochondria are involved in energy production through cellular respiration, and the nucleus houses the cell's genetic material and is the site of transcription where mRNA is synthesized. While each of these components is vital for cellular function, it is the ribosomes that are directly responsible for building proteins, making them the correct answer to the question.