Explanation:

- The speed of the boat in still water is 16 km/h.

- The time for 18 km upstream equals the time for 30 km downstream.

- Let the speed of the stream be x km/h.

- Upstream speed = 16 - x km/h.

- Downstream speed = 16 + x km/h.

Using the relation:

$$

\frac{18}{16-x} = \frac{30}{16+x}

$$

Solving the equation:

- Cross-multiply:

$$

18(16+x) = 30(16-x)

$$

- Expand and simplify:

$$

288 + 18x = 480 - 30x

$$

- Combine and solve for x:

$$

48x = 192 \implies x = 4

$$

- Option 3: 4 km/h is the correct answer.