Which is secreted by osteoblasts during bone deposition?

Collagen fibers are secreted by osteoblasts during bone deposition.

Explanation:

Osteoblasts are the bone cells which are primarily involved in bone formation.  

Osteoblasts first forms the osteoid with unmineralized collagen fiber or the procollagen molecule. It also synthesizes the extracellular bone matrix (organic and inorganic). These then form the collagen type I fibers. These fibers polymerize and forms the collagen strands.

The type I collagen fibers constitute for the majority of the protein content in bones because collagen is a fibrous protein.  

Later mineral deposition takes place on this collagen. Collagen provides both flexibility and strength to the bones.