Answer : 2. "320 ml"
Explanation :

Answer: B) 320 ml Explanation: Here total parts of milk and water in the solution is 6+2 = 8 parts. 1part = 640/8 = 80   old mixture contains 6parts of milk and 2 parts of water(6:2).   To get new mixture containing half milk and half water, add 4parts of water to the old mixture then 6:(2+4) to make the ratio same. i.e, 4 x 80 = 320 ml.