- 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.**

✅ **YOUR ANSWER IS RIGHT**