Made a mistake in I-539 application for H4 Extension. Options?

Hello, I need help with a mistake made in I-539 application for h4 extension. My wife accidentally answered ‘Yes’ to this question - ‘Were you granted a duration of Status (D/S) ?’ where the correct answer should have been ‘No’. Does anyone know if it’s a serious error ? I have already called the USCIS call center and they asked me to send a letter. So does sending a letter can fix this mistake or I have to wait for RFE ?

You should send a letter indicating your mistake. If they do not honor it, then they may issue an RFE. So, it is subjective hard to say…