The solid linear cytoskeletal elements having a diameter of 6 nm and made up of a single type of monomer are known as:
There are 3 main cytoskeletal elements.
Microfilaments: 6-7 nm wide
Intermediate filaments: 8-12 nm wide
Microtubules: 25 nm wide.
Thus, we can conclude that microfilaments , which is 6-7 nm wide and is made of actin monomers are the answer.