Are monthly returns required to be filed by the person opting to pay tax under the composition scheme

No. Such persons need to electronically file quarterly returns in Form GSTR-4 on the GSTN common portal by the 18th of the month succeeding the quarter. For example return in respect of supplies made during July, 2017 to September, 2017 is required to be filed by 18th October, 2017.

(FAQ 11: COMPOSITION LEVY)