Least constancy of shape is exhibited by

The correct option is WBC (White Blood Cells).

Here is a step-by-step explanation of why White Blood Cells (WBCs) exhibit the least constancy of shape compared to other blood cells and epithelial cells:

1. Understanding "Constancy of Shape"
"Constancy of shape" refers to how rigid or fixed a cell's structure remains. A cell with high constancy retains a specific, defined shape throughout its lifespan. Conversely, a cell with the "least constancy" or low constancy is irregular, highly flexible, and capable of changing its shape dynamically.

2. Function and Mobility of White Blood Cells (WBCs)
White Blood Cells (also known as leukocytes) play a critical role in the body's immune defense system. To perform their functions effectively, they must be highly mobile and capable of moving out of blood vessels into surrounding tissues to fight infections. This process of squeezing through the tiny pores of capillary walls is known as diapedesis.

3. Amoeboid Movement
To facilitate diapedesis and engulf foreign pathogens (a process called phagocytosis), WBCs exhibit amoeboid movement. They constantly extend temporary cytoplasmic projections called pseudopodia, which causes their shape to change continuously. Because of this dynamic behavior, WBCs do not have a fixed shape.

4. Comparison with Other Options
Let's look at the other cell types mentioned in the options:
- RBC (Red Blood Cells): Mature mammalian RBCs have a relatively constant, specialized biconcave disc shape optimized for oxygen transport.
- Blood platelets: These are small, disc-shaped cell fragments. Although they can change shape when activated during clotting, they generally maintain a stable morphology in their inactive state compared to the active amoeboid changes of WBCs.
- Epithelial cells: These cells form protective sheets and linings in the body and are tightly packed with distinct, fixed shapes (such as squamous, cuboidal, or columnar) to maintain structural integrity.

Consequently, because they must actively and continuously alter their structure to migrate and engulf pathogens, WBCs exhibit the least constancy of shape.