Arrow a is indicating a(n) protein. a structure of a cell showing many types of proteins with different functions outside and inside the cell.
a. a protein marked a is a hollow cylinder comprising filaments, which connect to the cell membrane.
b. a protein marked b is located in the cytoplasm and has a specific binding site.
c. a protein marked c is a small spherical protein outside the cell. an integral membrane protein marked d is bonded to an oval green structure via a specific binding site of d on the side of the cytoplasm.
d. an integral membrane protein marked e consists of two subunits, which are tightly connected to each other.

Microtubules composed of tubulin protein.

Explanation:

The cytoskeleton is composed of three well defined filamentous structures: microtubules, microfilaments, and intermediate filaments. Each of these filamentous structures is a polymer of proteinic subunits united by weak, not covalent connections.

Microtubules are long hollo cylindric tubes with no ramifications of a diametre near to 25 nanometers. They are constituted by two subunits of tubulin protein which polymerize to form microtubules. These filaments shape the cell and are involved in intracellular transport.  Microtubules extend from an organizing center near the nucleus to the cellular surface.