Answer: Ribosome
Explanation: The ribosome is the cellular structure primarily responsible for synthesizing proteins by translating messenger RNA (mRNA) into polypeptide chains. While the nucleus contains the genetic material and is involved in the transcription of DNA to mRNA, and the smooth endoplasmic reticulum is involved in lipid synthesis and detoxification, the ribosome is specifically the site of protein production. Mitochondria are primarily involved in energy production through ATP synthesis.