Explanation:

- Tap A can fill the tank in 8 hours. So, its rate is $\frac{1}{8}$ of the tank per hour.

- Tap B can empty the tank in 12 hours. So, its rate is $-\frac{1}{12}$ of the tank per hour (since it empties).

- When both taps are opened, the combined rate is $\frac{1}{8}-\frac{1}{12}$.

- Calculate the combined rate:

$$

\frac{1}{8} - \frac{1}{12} = \frac{3}{24} - \frac{2}{24} = \frac{1}{24}

$$

- Therefore, both taps together fill the tank in 24 hours.

- Option 3: 24 hours is correct.