Abone thickens select one:
a. as a result of cell division in the medullary cavity.
b. due to increased production of bone matrix by osteoclasts.
c. as a result of increased activity within the epiphyseal plate.
d. as compact bone is deposited beneath the periosteum of the diaphysis.

Option D.

Explanation:

The process of bone thickening starts with the formation of a template of the hyaline cartilage diaphysis from chondrocytes. Due to developmental signals the bone matrix begins to calcify. This calcification allows the prevention of nutrition diffusion into bone matrix and opens the cavities in diaphysis cartilage. Blood vessels enters into the cavities and  osteoblasts and osteoclasts modifies calcified cartilage matrix into spongy bone.  Osteoclasts allows the break down of spongy bone and forms a medullary cavity in diaphysis. Irregular connective tissue forms a periosteum (covers compact bone) around the bones and allows the attachment of  bone to tissues, tendons, and ligaments. The bone then continues to grow and get thick.

Hence, the correct option is D.