ANS:D - 22.5 litres

For thin structures subjected to wetting and drying, the water-cement ratio should be 0.45.
For mass concrete structures subjected to wetting and drying, the water ratio should be 0.55.
For thin structures which remain continuously underwater, the water-cement ratio by weight should be 0.55.
For massive concrete structures which remain continuously underwater, the water-cement ratio by weight should be 0.65. if the water-cement ratio is specified as 0.45, the quantity of mixing water required per 50-kg bag of cement is 0.45 - 50 = 22.5 lt or kg. W/c,=.45.
W = .45 * 50,
W = 22.5 Min water required for complete hydration of cement is 42% by wt of cement
So, water req is 42*50 = 21. Water for complete hydration req =23%by wt.
But for interapted voids 15%more water will be req so 15+23=38%,
Becouse we dont have any w/c ratio so for one bag water req will be =0.38*50=19litter.
So no ans match but nearby 20.5 is correct.