The right answer is balls (globules).
Microtubules are constituent fibers of the cytoskeleton, as are actin microfilaments and intermediate filaments.
The microtubules are formed of tubulin dimers (which have a spherical shape) each consisting of two subunits, tubulin α and tubulin β, linked by non-covalent bonds.
Actin, the main component of microfilaments, is a bi-globular protein that plays an important role in architecture and cell movement. It is present in all the cells of the body (ubiquitous protein), and especially in the muscular cells.
