On its own, G-actin (globular actin) is not likely to nucleate and begin to form chains of F-actin (fibrous actin); therefore, it is useful to have proteins to help the nucleation process. One of these protein complexes is Arp2/3. Arp2/3 is especially known for its function of nucleating actin chains that branch off of previously established actin chains. Myosins are motor proteins that interact with actin chains to perform various functions, such as muscle contraction and transporting vesicles. Cofilin is a protein that binds G-actin monomers and helps them dissociate from F-actin.

Each choice describes a distinct function of the cytoskeleton. The cytoskeleton is involved in supporting various organelles, helping to anchor them in various locations around the cell and maintaining their shape and integrity. It also has the important function of helping with vesicle trafficking by associating with various motor proteins that carry vesicles from one part of the cell to another. The cytoskeleton is also a part of several different types of cell junctions (e.g. adherens junctions). Finally, the cytoskeleton is also an important part of various motile structures, such as cilia and flagella. 

The three major components of the cytoskeleton in cells are microtubules, intermediate filaments, and microfilaments. Microtubules are the larger filaments and make up the mitotic spindle, as well as flagella and cilia. Intermediate filaments are used in structural maintenance.

Microfilaments are the smaller filaments and make up the polymerized actin filament in muscle fibers.

Microfilaments and microtubules are both polarized, and can be used in vesicular transport. Intermediate filaments lack polarity and serve only structural functions.

Intermediate filaments form a veil right next to the nuclear membrane, are of intermediate thickness with respect to the other two cytoskeletal filaments, and they almost exclusively play structural roles. Actin filaments brace cells against surfaces and allow contractions in striated muscle. Also, actin filaments provide structural support and have a role in determining cell shape. Microtubules form the mitotic spindle and comprise cilia and flagella. They are also the "freeways" on which motor proteins move and transport vesicles throughout the cell.

GDP-GTP cycling can regulate a number of these cytoskeletal components, although GTP-binding is especially crucial to microtubule polymerization. In addition, microtubule polymers are comprised of alpha and beta tubulin, while this is not the case for the other components listed. Microtubules are the only choice that fit all of the criteria posed in the question.