The correct answer is C. Thicker centrum.

Lumbar vertebrae have several distinct features that differentiate them from thoracic vertebrae. Some of these features include:

- Thicker and more robust centrum (vertebral body)

- Shorter and more sturdy transverse processes

- No costal facets (since lumbar vertebrae do not articulate with ribs)

- Larger and more triangular vertebral foramen

In contrast, thoracic vertebrae typically have:

- Longer and thinner transverse processes

- Costal facets for articulation with ribs

- Smaller vertebral foramen

So, while options A and D are partially correct, the most distinctive feature of lumbar vertebrae is their thicker centrum.