Which of the following structures is not found in some prokaryotic cells?
a) axial filament
b) flagellum
c) pilus
d) cilium
e) peritrichous flagella

The answer would be:

Cilium

Here is more about the structures of prokaryotic cells:

Axial filament is a bundle of flagella that are wrapped around the cell's body.

Flagellum is what the prokaryotic cell has that is whip-like in structure. It helps with the motility of the cell.

Pilus is a hair-like structure that are found on the surface of prokaryotic cells. They help the cell attach itself to surfaces.

Peritrichous flagella are several flagella that can be found all over some prokaryotic cells.