The correct answer is A. GSTR-1.
GSTR-1 is the first return filed under GST. It is a monthly return that must be filed by all registered taxpayers. The return must be filed by the 10th of the following month.
GSTR-1 is a summary of all the sales and purchases made by the taxpayer in the previous month. It includes details of the goods and services sold, the value of the sales, the taxes collected, and the taxes paid.
GSTR-1 is an important document for taxpayers. It is used to calculate the GST liability of the taxpayer and to file the GST returns. It is also used by the government to monitor the GST compliance of taxpayers.
The other options are not correct.
GSTR-3A is a quarterly return that must be filed by taxpayers who are registered under the composition scheme.
GSTR-2 is a monthly return that must be filed by taxpayers who are registered under the regular scheme and who have opted for the monthly return filing option.
GSTR-3B is a monthly return that must be filed by taxpayers who are registered under the regular scheme and who have opted for the quarterly return filing option.
GSTR-4 is an annual return that must be filed by all registered taxpayers.