To make a snowflake, a square sheet is folded in half, in half again, and then along the diagonal of the small square. The two shaded pieces are cut away. Which figure shows the opened sheet?

Unfold in reverse: mirror the cuts in the diagonal crease, then in the horizontal crease, then in the vertical crease. The cut on the folded edge opens into diamonds along both centre lines, and the notch on the outer edge appears twice on every side of the sheet: (b).