What are flagella and cilia of eukaryotic cells made of?

The correct option is tubulin.

To understand why this is the correct answer, let us break down the structure and composition of eukaryotic flagella and cilia step-by-step:

1. What are flagella and cilia?
Flagella and cilia are hair-like, motile appendages that project from the surface of various eukaryotic cells. They play vital roles in cellular movement, moving fluids over the cell surface, and sensory functions.

2. The internal structure (The Axoneme)
The core structural component of both eukaryotic cilia and flagella is called the axoneme. When viewed in cross-section under an electron microscope, the axoneme of motile cilia and flagella typically shows a characteristic "9 + 2" arrangement. This arrangement consists of nine outer doublets surrounding a central pair of single structures.

3. Microtubules are the building blocks
These doublets and central pairs are actually microtubules, which are key components of the cell's cytoskeleton. Microtubules are hollow tubes constructed from dimers of globular proteins.

4. The protein component: Tubulin
The specific protein that polymerizes to form microtubules is tubulin (specifically $\alpha$-tubulin and $\beta$-tubulin heterodimers). Therefore, because the structural framework of flagella and cilia consists of microtubules, they are primarily made of the protein tubulin.

5. Why the other options are incorrect:
• Lamin: Lamins are fibrous proteins that form the nuclear lamina, a structural framework inside the nuclear envelope.
• Desmin: Desmin is an intermediate filament protein found mainly in muscle cells, helping to integrate sarcolemma, Z-discs, and the nuclear membrane.
• Keratin: Keratins are intermediate filament proteins that provide structural support to epithelial cells, making up structures like hair, nails, and the outer layer of skin.