Proteins are transported via axoplasmic transport at slow rates (0.5 to 2 mm per day) and fast rates (200 to 400 mm per day). the difference in transport rate directly results from:
a. the difference in the time spent per day in transport by kinesin on microtubules.
b. the site where the protein is synthesized
c. the substrate molecule used by the motor protein
d. the motor protein used to transport the proteins

Option (a).

Explanation:

Protein is one of the most important biomolecules and a basic building block of the body. Axoplasmic transport of protein includes the transportation of protein to and from the neuron.

Axoplasmic transport of protein shows the difference in transportation rate because different proteins spent different time on microtubules  during transport by the kinesin molecule.

Thus, the correct answer is option (a).