In which part of the cell does protein synthesis occur?

Protein synthesis occurs primarily at the ribosomes, which are often referred to as the "protein factories" of the cell. Ribosomes can be found floating freely in the cytoplasm or attached to the endoplasmic reticulum, which can be identified as rough ER due to the presence of these ribosomes. During protein synthesis, ribosomes translate messenger RNA (mRNA) into polypeptide chains, which then fold into functional proteins.

While the nucleus is involved in the initial stages of gene expression—transcribing DNA into mRNA—actual protein synthesis does not occur there. Mitochondria are primarily known for their role in energy production rather than synthesizing proteins, although they do have their own ribosomes and synthesize a limited number of proteins. The endoplasmic reticulum, particularly the rough ER, plays a supportive role in protein processing and transport rather than the actual synthesis, which is fundamentally carried out by ribosomes. Hence, the ribosome is the correct answer as it is directly responsible for the process of building proteins based on the genetic instructions carried by mRNA.