Shared Cellular Structures Steps:

• Define prokaryotes as cells without nucleus or membrane-bound organelles, like bacteria.
• Define eukaryotes as cells with nucleus and organelles, like animal and plant cells.
• Identify universal features: both types have plasma membranes for boundary and selective permeability.
• Confirm ribosomes as site of protein synthesis in cytoplasm of both cell types.

Why A is correct:

• Cell membranes enclose and regulate both prokaryotic and eukaryotic cells; ribosomes enable protein production in all cells.

Why the others are wrong:

• B: Nucleus and mitochondria are membrane-bound and exclusive to eukaryotes.
• C: Nucleus is absent in prokaryotes, though cell membrane is shared.
• D: Mitochondria are eukaryotic-only organelles.

Final answer: A