Ans. is 'b' i.e., Osteoclasts o The cells responsible for bone formation are osteoblasts and the cells responsible for bone resorption are osteoclasts. Cell in Boneo The cells of bone are :-OsteoblastsOsteoblasts form a cell layer over bone surfaces. The bone is essentially enveloped by the osteoblasts, since the cells are in close contact with one another and tight junctions and gap junctions have been observed. Thus, the osteoblastic layer controls the transport of materials from the extracellular space to the osteoid seam and mineralization in front. Osteoblasts are responsible for synthesis of major proteins of bone and for mineralization of bone.It plays a central role in osteoclastic function (i.e. involved in initiation & control of osteoclastic activity). Thus osteoblasts and not osteoclasts have specific surface receptors for agent, such as 1, 25-dihydroxy vitamin D and parathyroid hormone0 So osteoclasts in culture, w hich are therefore not in contact with osteoblasts, do not respond to these agents.OsteoclastsOsteoclasts are found in sites in which bone is being remodeled. These are multinucleatedgiant cells and formed by fusion of mononuclear cells (monocytes0 & macrophages). These cells are the principal mediator of bone resorption0 The characteristic feature is the area of infolded plasma membrane known as ruffled border wrhich is surrounded by an organelle free clear zone through which osteoclast attaches to bone & wrhich is the site of bone resorption.OsteocytesApproximately 10% of osteoblastic population become enclosed in the developing matrix and are then referred to as osteocytes.