Which cytoskeletal element has a diameter of about 10 nanometres?

Correct option is D

Intermediate filaments are part of the cytoskeleton and have a diameter of about 10 nm, placing them between microfilaments (6–7 nm) and microtubules (25 nm). They provide mechanical support and structural stability to the cell.
Explanation of options: (a) ✘ Centrioles are not measured by filament thickness and have complex cylindrical structures. (b) ✘ Microfilaments are the thinnest cytoskeletal elements (~6–7 nm). (c) ✘ Microtubules are the thickest (~25 nm), not 10 nm. (d) ✔ Intermediate filaments are the correct cytoskeletal structure with a diameter of ~10 nm.