Explanation:

- Initial mixture composition: The 30-liter solution contains milk and water in a ratio of 2:1. Thus, milk = 20 liters and water = 10 liters.

- Mixture removal: 12 liters are removed, maintaining the ratio of 2:1. Therefore, 8 liters of milk and 4 liters of water are removed.

- Solution status after removal: 12 liters of milk and 6 liters of water remain, totaling 18 liters.

- Pure milk is added: 12 liters of pure milk is added, making the total milk in the mixture 24 liters and water still 6 liters.

- Final ratio: The final ratio of milk to water is 24:6, simplifying to 4:1.

Given these calculations:

- Option 1, 1:4 - Incorrect, does not match the final simplified ratio.

- Option 2, 4:1 - Correct as this matches the final milk to water ratio.

- Option 3, 3:1 - Incorrect, not matching the calculations.

- Option 4, 1:3 - Incorrect, opposite of initial ratio.

- Option 5, None of these - Incorrect, as we've found the correct answer.

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