The solid linear cytoskeletal elements having a diameter of 6 nm and made up of a single type of monomer are known as
a) Microfilaments
b) Intermediate filaments
c) Lamins
d) Microtubules

Hint: Mostly cytoskeletal elements are made up of either microfilaments or microtubules. Microtubules are 20-25 nm in diameter. They are made up of two strands of actin. They help in movement, cell motility etc.

Complete answer:
Microfilaments are the solid linear cytoskeletal elements made up of a single type of the cytoskeleton and are found in the cytoplasm of the eukaryotic cells. The eukaryotic cell is composed of a network of protein fibres that is known as the cytoskeleton. The cytoskeleton is unique to eukaryotic cells. It is a dynamic three dimensional structure that fills the cytoplasm. The primary types of fibres comprising the cytoskeleton are microfilaments, microtubules, and intermediate filaments.
a) Microfilaments: Microfilaments are fine, thread like protein fibers, 3-6 nm in diameter. They are composed predominantly of a contractile protein called actin. These are flexible and relatively strong.
b) Intermediate filaments: Intermediate filaments are about 10nm diameter and provide tensile strength for the cell. They are the numerous microscopic protein fibres that form cytoskeleton.
c) Lamins: lamin is the protein present in intermediate filaments.
d) Microtubules: Microtubules are cylindrical tubes. They are composed of subunits of the protein tubulin. These subunits are termed alpha and beta. Microtubules are of 24 nm diameter and of variable length.

So the correct answer is option A.

Note: The long fibers of the cytoskeleton are polymers of subunits. Each microfilament consists of two helically twisted strands, each comprising a chain of globular subunits of actin proteins. Microfilaments are the thinnest cytoskeleton.