The question is poorly designed as it seems to mislead the student into thinking that the structural adaptions were to cope with the scarcity of water.
(4) is correct
a : Long eyelashes keep out the sand from their eyes
b: Camel humps help to store fat to replenish itself when food is scarce
c: Producing concentrated urine makes sense in an arid environment as waster is preserved
d: Camels can actually drink a lot of water in one sitting (up to 20 gallons)