Opting for quarterly GST returns requires online election within the prescribed window; turnover threshold triggers monthly filing. Registered persons must elect to opt for furnishing quarterly returns electronically on the common portal within the specified window before the quarter; the quarterly option remains in effect for future tax periods unless the person becomes ineligible, elects monthly filing, or has not furnished the last due return at the time of election. Where aggregate turnover in the current financial year exceeds the notified threshold, the person must furnish returns monthly from the first month of the quarter succeeding the quarter in which the threshold was exceeded.