Ans. is 'a' i.e osteopetrosis Osteopetrosis (also known as Marble bone disease, Albers- Schonberg disease)Defect is in bone resorption due to abnormal function of osteoclasts.Defective bone resorption results in uniformly thickened dense bones, often without distinction between cortical and cancellous regions. The bones are dense but brittle therefore known as marble bone disease.Inheritance is both AD & AR*.The AR form dies early in the life.The AD form is of milder variety.Clinical features in osteopetrosis are caused byan increased tendency to fractures and osteomyelitis.Encroachment of the marrow space leading to anaemia and extramedullary hematopiesis- hepato splenomegaly & cranial nerve compression.