Here's why:

* Ribosomes are the cellular machinery that reads mRNA (messenger RNA) and uses it to assemble amino acids into proteins.

* Rough endoplasmic reticulum (RER) is a network of interconnected membranes within a cell. It's called "rough" because its surface is studded with ribosomes.

* Proteins synthesized by ribosomes attached to the RER are often destined for:

* Secretion from the cell (e.g., hormones, enzymes)

* Incorporation into cell membranes

* Use within other organelles

Therefore, the RER is the structure with ribosomes on it that is involved in making proteins.