Explanation:

- The Bhopal gas tragedy is considered the world's (1) largest industrial disaster.

- Forty-two tonnes of methyl isocyanate (2) leaked from the steel containers.

- The correct preposition for blank No. 3 is important to indicate the location in relation to the Union Carbide factory.

- Option 1: Across - This could imply from one side to another or opposite, which isn't contextually relevant.

- Option 2: At - This suggests a specific point or location, fitting the context of where the leak occurred.

- Option 3: Beside - This implies being next to something, which does not specify the accuracy needed.

- Option 4: Along - This suggests movement or a path, not fitting for a fixed location like a factory.

- Correct Answer: 2, at - This option is apt as it specifies the location relevant to the factory.