Explanation:

- The man rows from J to K and back, covering a total distance of 600 km in 15 hours.

- He takes the same time to row 9 km downstream as he does 3 km upstream.

- Let the speed of the boat in still water be $b$ km/h and the speed of the stream be $s$ km/h.

- Going downstream, the effective speed is $b+s$, and upstream, the effective speed is $b-s$.

Equation for the given condition:

- Time taken to cover 9 km downstream = time for 3 km upstream:

$$

\frac{9}{b+s} = \frac{3}{b-s}

$$

Simplification gives us:

- $3(b+s)=9(b-s)$

- Solving gives $b=2s$.

Total time equation:

- $\frac{300}{b-s}+\frac{300}{b+s}=15$

Plugging $b=2s$ into the equation:

- Solving gives $b=53.33$ km/h.

- Correct Answer: Option: 3 - 53.33 km/h