Explanation:

To find the speed of the train, use the formula:

$$\text{Speed}=\frac{\text{Distance}}{\text{Time}}$$

- Total Distance Covered:

- The train and the platform together total 2100 metres (900 + 1200).

- Time Taken:

- The train takes 70 seconds to cross this distance.

- Speed in m/s:

$$\frac{2100\text{m}}{70\text{s}}=30\text{m/s}$$

- Convert to km/h:

- Since 1 m/s equals 3.6 km/h:

$$30\text{m/s}\times 3.6=108\text{km/h}$$

Option 2: 108 km/h is the correct answer.