Answer:
Byte stuffing is an operation that transforms a sequence of data bytes that may contain reserved values such as packet delimiter into a potentially longer sequence that contains no occurrences of those values. The size of each frame to be transmitted in variable length framing may be different. Therefore, a sequence of bits is used as a delimiter to mark the end of one frame and the beginning of the next frame. A byte is stuffed in the message to differentiate from the delimiter.
The output after stuffing will be: A B ESC ESC C ESC ESC ESC FLAG ESC FLAG D