The correct answer is Ribosomes.

Both eukaryotic and prokaryotic cells share the common feature of having ribosomes. While eukaryotic cells possess membrane-bound organelles like the nucleus, mitochondria, and endoplasmic reticulum, prokaryotic cells lack these. However, the fundamental process of protein synthesis is present in both types of cells, and this is carried out by ribosomes. Ribosomes are the sites of protein synthesis, translating the genetic code from messenger RNA (mRNA) into polypeptide chains. These organelles are relatively small, composed of ribosomal RNA (rRNA) and proteins, and are found either freely floating in the cytoplasm or bound to the endoplasmic reticulum in eukaryotes. The structural differences between prokaryotic and eukaryotic ribosomes are relatively minor, leading to the possibility of targeting prokaryotic ribosomes with antibiotics without harming the host eukaryotic cells.

Let's eliminate the incorrect options:

• Cell organelle: This is too broad a term. Many organelles are unique to eukaryotes.
• Nucleus: Only eukaryotic cells possess a membrane-bound nucleus.
• Cell membrane: While both cell types have a cell membrane, the composition and structure may differ.

Therefore, ribosomes are the only organelle consistently present in both eukaryotic and prokaryotic cells, making them the correct answer.