In the given figure, which component has thin outer walls and highly thickened inner walls?

Based on the provided diagram of a stomatal apparatus, we can identify each labeled component as follows:

1. A represents the surrounding epidermal cell (or subsidiary cell).
2. B represents the guard cell (specifically pointing to the body containing chloroplasts).
3. C represents the highly thickened inner wall of the guard cell.
4. D represents the stomatal pore (aperture).

Guard cells are specialized cells that surround the stomatal pore and regulate its opening and closing. A characteristic feature of guard cells is the unequal thickening of their cell walls:
- The outer walls (facing away from the stomatal pore) are thin, light, and elastic.
- The inner walls (facing the stomatal pore) are highly thickened, rigid, and inelastic.

In the given diagram, the label C points directly to this thickened inner wall of the guard cell. Therefore, the component with thin outer walls and highly thickened inner walls is represented by C.

The correct option is C.