A nine month old boy parents presented to you with complaints of progressive lethargy, irritability & pallor since 6 months of age. Examination revealed severe pallor. Investigation showed Hb-3.8 mg%; MCV-58 fl; MCH-19.4 pg/cell. Blood film shows osmotic fragility is normal (target cells and normoblasts). X-ray skull shows expansion of erythroid marrow. Which of the following is the most likely diagnosis?