Which type of ribosomes are found attached to the endoplasmic reticulum?

Bound ribosomes are the correct answer because they are specifically those ribosomes that are attached to the endoplasmic reticulum (ER), converting it into rough endoplasmic reticulum (rough ER). These ribosomes synthesize proteins that are typically destined for secretion outside the cell, incorporation into the plasma membrane, or for lysosomes. The attachment of ribosomes to the ER is an important aspect of cellular organization and function, as it allows for the co-translation translocation of proteins into the ER lumen where they undergo folding and post-translational modifications.

Free ribosomes, on the other hand, float freely in the cytoplasm and primarily synthesize proteins that function within the cytosol. This distinction between free and bound ribosomes is crucial for understanding protein synthesis locations and the pathway proteins take within a cell. The presence of both types allows cells to efficiently manage protein synthesis according to their specific needs.