Deficiency is the developing world’s most common cause of preventable brain damage.
a. iron
b. vitamin a
c. iodine
d. calcium

The answer is vitamin a
vitamin a deficiency (vad) is a  lack   of vitamin a in humans. it is common in poorer countries but rarely seen in more developed countries.
nyctalopia (night blindness ) is one of the first signs of vad.
it is the major cause of preventable brain damage,