Imagine a mirror placed along one of the two diagonals of each grid. Four grids would look the same in a suitable diagonal mirror. Which grid would not?

Fold each grid along a diagonal. In (a), (b), (c) and (d) the shaded cells on one side of some diagonal land exactly on shaded cells on the other side. Grid (e) is balanced about the upright centre line, not about either diagonal: folding along a diagonal moves its top-right cell onto an empty cell. So (e) is the odd one out.