Explanation:

- GSTR-1: This is the correct return used to file outward supplies under the GST Act. This return captures the details of all outward supplies of goods and services made by a taxpayer.

- GSTR-2: This return was meant for reporting inward supplies received by a taxpayer but it has been suspended by the government and is not currently in use.

- GSTR-3B: This is a summary return for payment of GST and not specifically for outward supplies. It is filed monthly.

- GSTR-3: This was an earlier proposed return for monthly summary of inward and outward supplies, input tax credit, and payment of tax. It is currently not in use.