Explanation:

- Initially, the mixture has water and alcohol in the ratio 2:1.

- Let's assume the initial amounts are 2x liters of water and x liters of alcohol.

- A total of 3x liters is the mixture.

- If 12 liters of the mixture is removed, you remove \( \frac{12}{3x} \times 2x \) liters of water and \( \frac{12}{3x} \times x \) liters of alcohol.

- After replacing the 12 liters with water, the new ratio becomes 4:1.

- Solving equations:

- Water after change: \(2x - 8 + 12 = 2x + 4\)

- Alcohol after change: \(x - 4\)

- New ratio equation: \( \frac{2x + 4}{x - 4} = 4 \)

- Solving this:

- \(2x + 4 = 4(x - 4)\)

- \(2x + 4 = 4x - 16\)

- \(20 = 2x\)

- \(x = 10\)

- The initial amount of alcohol is 10 liters.

- Correct answer: Option 4: 10 liters