The disease that occurs due to protein deficiency in children is (a) Marasmus. Marasmus is a severe form of malnutrition characterized by inadequate intake of protein and calories. It leads to extreme weight loss, muscle wasting, stunted growth, weakened immune system, and other developmental issues. Unlike other conditions like (b) Pellagra (resulting from niacin deficiency), (c) Beri-Beri (resulting from thiamine deficiency), or (d) Rickets (resulting from vitamin D deficiency), Marasmus is specifically linked to insufficient protein intake, highlighting the critical importance of dietary protein for healthy growth and development in children. Therefore, the correct choice is (a) Marasmus.