The internal organ that premature babies sometimes haven't developed is the lungs. Option B
This is because a fetus is given an oxygen supply by its mother. If for some reason the fetus doesn't get an oxygen supply from its mother it will suffocate. Because, the lungs aren't needed in the womb they don't develop until the later days of pregnancy. The oxygen the fetus needs is provided through the umbilical cord. Just after birth, the umbilical cord is cut off and the oxygen supply stops, and the baby usually clears its esaphogus of mucus by screaming. Then, the the baby starts to breath.