**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**