The organelle that functions as the site of protein synthesis in a cell is the ribosome.

Here's why:

* Ribosomes are complex structures made of ribosomal RNA (rRNA) and proteins. They are found in both prokaryotic and eukaryotic cells.

* Protein synthesis is the process of creating proteins from genetic information encoded in DNA. This information is first transcribed into messenger RNA (mRNA), which then travels to the ribosomes.

* At the ribosomes, the mRNA is translated, meaning the genetic code is read and used to assemble amino acids into a specific protein sequence.

So, the ribosomes are essentially the "protein factories" of the cell!