Concept: The question tests on Mixtures and Alligations
Solution:
729 litres contain milk and water in the ratio 7 : 2
Amount of Milk = (7 / 9) * 729 = 567
Amount of Water = (2 / 9) * 729 = 162
Let us add x liters of water. The amount of water in the solution = 162 + x
The new ratio = 7 : 3
Therefore 567162+x=73567162+x=73
567 * 3 = (162 + x) * 7
1701 = 1134 + 7x
7x = 567
x = 81
Option D