Explanation:

- The Union Cabinet approved India's first bank merger involving Bank of Baroda, Vijaya Bank, and Dena Bank.

- This merger aimed to create a stronger financial entity to enhance banking operations.

- Option 1: 1 January 2019 - This date is incorrect for when the merger came into effect.

- Option 2: 1 March 2019 - This date is also not accurate for the merger's commencement.

- Option 3: 1 April 2019 - This is the correct date when the merger came into force.

- Option 4: 1 February 2019 - This is not the date of the merger effect.

- Option 5: 28 February 2019 - This date does not mark the merger's start.