Four of the tile designs below can be folded along a straight line so that the shaded squares on the two halves match exactly. Which design is the odd one out?

Find a fold (mirror) line for each: (a) folds along the horizontal middle line, (b) along the diagonal from top-left to bottom-right, (c) along the vertical middle line and (e) along the diagonal from top-right to bottom-left. (d) has no mirror line at all, so (d) is the odd one out.