A lap forms when metal that is already flowing in one direction gets folded back onto a surface instead of properly filling the die. This usually happens where the die has a tight or sharp corner that resists smooth flow, so a thin tongue of metal buckles and presses flat against an adjacent surface without ever fusing to it, since both surfaces are already partly oxidised and cooled. The result is a crack like seam sitting right at the surface. Since this all comes down to the metal being guided badly through the die instead of flowing evenly, the cause is improper metal flow, option (A).