Explanation:

- To find the speed in km/h, start by converting the distance and time into suitable units.

- The person crosses 1800 meters in 6 minutes.

- Convert 1800 meters to kilometers: $$1800\text{m}=1.8\text{km}$$.

- Convert 6 minutes to hours: $$6\text{min}=\frac{6}{60}\text{h}=0.1\text{h}$$.

- Speed in km/h is calculated using the formula: $$\text{Speed}=\frac{\text{Distance}}{\text{Time}}=\frac{1.8\text{km}}{0.1\text{h}}=18\text{km/h}$$.

- Option 3: 18 km/h is the correct answer.