Explanation:

- To solve this problem, let the speed of the boat in still water be $b$ km/h and the speed of the stream be $s$ km/h.

- Downstream speed = $b+s$, upstream speed = $b-s$.

- For the first condition, equation is:

$$

\frac{10}{b+s} + \frac{8}{b-s} = \frac{49}{60} \text{ (hours)}

$$

- For the second condition, equation is:

$$

\frac{12}{b+s} + \frac{4}{b-s} = \frac{42}{60} \text{ (hours)}

$$

- Solving these two equations simultaneously can get you both $b$ and $s$.

- After solving, the stream speed $s$ is found to be 2 km/h.

- Hence, option 3 is correct.