Which method is used for filling the ballast tank?

The method involving free flow from the sea chest through the ballast main with an electronic driven pump is the correct approach for filling the ballast tank. This method is efficient and allows for better control of the filling process, ensuring that the ballast tank can be filled rapidly and safely.

Using a steam-driven pump, while functional in some scenarios, is less common for filling ballast tanks due to the complexities and potential inefficiencies involved with steam systems in modern naval vessels. Additionally, manual filling with buckets is impractical for large tanks, as it would be time-consuming and labor-intensive. Vacuum suction from ballast tanks is typically used for emptying the tanks rather than filling them, as it relies on creating a pressure differential rather than employing a flow of water into the tanks. Thus, the electronic driven pump utilizing free flow from the sea chest combines effectiveness and ease of operation for managing ballast operations.