The correct answer is a) fimbriae.
Fimbriae are short, hair-like structures that protrude from the surface of prokaryotic cells, particularly in bacteria. They play a crucial role in the adherence of bacteria to surfaces and to each other, which is essential for colonization and infection processes. While some eukaryotic cells may have similar structures, such as cilia or pili, true fimbriae are distinct to prokaryotes.
On the other hand, cell walls are present in many eukaryotic organisms, such as plants and fungi, although their composition differs from those in bacteria. Both prokaryotic and eukaryotic cells can possess flagella for movement, and ribosomes are universally found in both types of cells for protein synthesis. Thus, fimbriae stand out as a feature unique to prokaryotic cells.