The solid linear cytoskeletal elements having a diameter of $7$ nm and made up of a single type of monomer are known as:

Microfilaments or actin filaments are long cytoskeletal filaments present in the cytoplasm of the eukaryotic cells. These filaments are primarily composed of polymers of actin and are $7$ nm in length. The globular monomeric actin polymerizes to form the filamentous F-actin.