The primary cilium:

a. is nonmotile because it lacks the "central pair" of microtubules.
b. is relatively susceptible to microtubule destabilizing drugs like colchicine.
c. has no known role in humans.
d. None of the answers is correct.

The correct answer is a. is nonmotile because it lacks the "central pair" of microtubules.

Explanation:

Microscopic sensory antennae which is present in the cell of many vertebrate tissue and is used to gather the information around the surrounding is called primary cilia. These cilia are non-motile because it lacks the central pair of microtubules.

It is also seen that primary cilia lacks dynein which is a motor protein that is needed for motility in cilia. Some of the cilia are very short and do not grow /project beyond the surface of cell. Therefore the correct answer is a.