Which of these is the primary site of protein synthesis?

Ribosome

Explanation:

Ribosome is a sub cellular structure, that is involved in protein synthesis.

Mitochondria is a sub cellular structure, where cellular respiration takes place, therefore it is called the energy house of the cell.

Cytoplasm is a jelly like structure in which sub cellular structures are found.

Nucleus is the main part of the cell in which all cellular activity is controlled by.

It contains DNA and RNA.

Nucleus is mostly known for it's thread like structure that contains hereditary information which is passed from parents to offspring.

Hope it helps :)