The solid linear cytoskeletal elements having a diameter of $6\text{nm}$ and made up of a single type of monomer are known as:

Microfilaments (actin filaments) are solid, linear structures with a diameter of about $6-7\text{nm}$ and are composed of a single type of monomer called G-actin. Microtubules are hollow and much thicker ($25\text{nm}$), while intermediate filaments have a diameter of $8-12\text{nm}$.