Explanation:

- All three pipes, A, B, and C, can fill the cistern together in 6 hours.

- In 1 hour, A, B, and C together will fill $\frac{1}{6}$ of the cistern.

- After working together for 2 hours, they fill $2\times \frac{1}{6}=\frac{1}{3}$ of the cistern.

- This means $\frac{2}{3}$ of the cistern still needs to be filled.

- Pipes A and B together fill the remaining $\frac{2}{3}$ of the cistern in 8 hours.

- In 1 hour, A and B together fill $\frac{1}{8}\times \frac{2}{3}=\frac{1}{12}$ of the cistern.

- Thus, in 1 hour, C fills $\frac{1}{6}-\frac{1}{12}=\frac{1}{12}$ of the cistern.

- Therefore, Pipe C alone can fill the cistern in 12 hours.

- Option 3: 12 hours is the correct answer.