Final answer:
The crucial difference between cilia and primary cilia lies in their 9+2 microtubule arrangement necessary for motility. The central pair of MTs is key for this functionality, underscoring the importance of microtubule organization and refuting the idea that the basal body is irrelevant.
Step-by-step explanation:
The difference between cilia and primary cilia gives us insights into their structure and function. Both have an arrangement of microtubules (MTs) known as a "9 + 2 array", which is essential for their motility. This array consists of nine outer doublet microtubules surrounding two central single microtubules. The presence of the central pair is particularly important, as it suggests that option 3, "the central pair of MTs in an axoneme is required for motility," is the correct answer to the student's question. This distinction emphasizes that the arrangement of MTs is crucial for the function of cilia and disproves option 1. Moreover, this organization also demonstrates that MT doublets can share functions with MT triplets, which partially supports option 2. However, the central pair differentiates the motility capabilities of full cilia from non-motile primary cilia. Lastly, the basal body is critical in providing a foundation for MTs, conflicting with option 4. Hence, based on the provided information, the central pair's significance is the central theme, highlighting the importance of structure in the function of these organelles.