Solution:
Let's assume that the milk sample has 100 ml of liquid. Out of this, 50 ml is water and the remaining 50 ml is milk.
If 1/3rd of this milk is added to an equal amount of pure milk, then we can calculate the new water content in the mixture as follows:

Step 1: Finding the amount of milk in 1/3rd of the milk sample
1/3rd of the milk sample = (1/3) * 100 ml = 33.33 ml (approx.)

Step 2: Adding this milk to an equal amount of pure milk
The amount of pure milk added = 33.33 ml
Total milk in the new mixture = 33.33 ml + 33.33 ml = 66.66 ml (approx.)

Step 3: Finding the water content in the new mixture
Total water in the new mixture = 50 ml (water in the milk sample) + 0 ml (water in the pure milk) = 50 ml
Total liquid in the new mixture = 66.66 ml (milk added to the pure milk)
Percentage of water in the new mixture = (50 / 66.66) * 100 = 75%

Step 4: Finding the final water content in the new mixture
Since we know that the final water content is 25%, we need to subtract the excess water (25% - 75% = -50%) from the mixture.
To do this, we need to remove some of the pure milk from the mixture.
Let's assume that we remove x ml of pure milk from the mixture.
The new water content in the mixture can be calculated as follows:

Total water in the new mixture = 50 ml (water in the milk sample) + 0 ml (water in the pure milk) = 50 ml
Total liquid in the new mixture = (66.66 - x) ml (milk added to the pure milk and some of it removed)
Percentage of water in the new mixture = (50 / (66.66 - x)) * 100 = 25%

Solving for x, we get:

(50 / (66.66 - x)) * 100 = 25%
50 = 16.665 - 0.25x
0.25x = 16.665 - 50
x = 133.32 ml

Since we cannot remove more milk from the mixture than what we added (33.33 ml), we can remove only 33.33 ml of pure milk from the mixture.

Therefore, the new mixture will contain 33.33 ml of milk added to 33.33 ml of pure milk, resulting in a water content of 25%. Hence, the correct answer is option A.