If you are a US Citizen and your mother entered legally, then the supplement A is not required. The fact that it is being requested indicates that either USCIS made a mistake, or you did by not submitting required evidence or filling the I-485 wrong. Regardless, you must respond to the request.
You should retain an experienced immigration lawyer to review all the facts and advise you accordingly.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship.
I do not understand why you did not file the I-130 and I-485 all together. In any event with legal entry she does not have to pay the penalty or file form I485A
Please be advised this answer is for informational purposes only and no attorney-client relationship has been formed. You should also see a local attorney for legal advise before acting on any of the statements given to you online as there may be aspects of your case that are not obvious by the question asked online.
Your mother is considered an immediate relative and entered the United States pursuant to a visa and inspection. As such, form I-485 supplement is not required.
You should consult with an immigration attorney to assist with your response to the RFE.
Choquette Law Group at 206.269.1200 or firstname.lastname@example.org. The statement above is general information only. It does not constitute legal advice and does not create an attorney/client relationship. In order to receive legal advice you will need to provide all information relevant to your case and schedule a free consultation.