Letter received from it

MY FATEHR IS AN INCOME TAX ASSESSE FROM 1990’S HE FILING IT REGULARLY . BUT HE RECEIVED LETTER FROM INCOME TAX TWO DAYS BACK THAT HE WAS NOT FILED IT FROM 2011-14 AND HE SHOULD REPLY FOR THAT LETTER . BUT HE ALREADY SUBMITTED RETURNS FOR THAT PERIOD AND HAVING COPY FOR THAT RETURN FILING . THIS YEAR 2015-16 HE FILED RETURNS IN EFILING FOR THE PREVIOUS YEARS HE FILED IN MANUAL . WHAT SHOULD WE DO