Explanation:

- A boat moves 42 km in 6 hours upstream.

- Upstream speed: $\text{Speed}=\frac{\text{Distance}}{\text{Time}}=\frac{42\text{km}}{6\text{hours}}=7\text{km/h}$.

- The stream's speed is 2 km/h.

- Therefore, the boat's own speed in still water = $7\text{km/h}+2\text{km/h}=9\text{km/h}$.

- Downstream speed is $9\text{km/h}+2\text{km/h}=11\text{km/h}$.

- The boat travels downstream for 12 hours.

- Distance covered downstream = $11\text{km/h}\times 12\text{hours}=132\text{km}$.

- Option:1, 156 km

- Option:2, 120 km

- Option:3, 132 km

- Option:4, 144 km

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