Which answer figure can be traced, in the same size and the same way up, somewhere inside figure X?

X has both diagonals, a diamond joining the midpoints of the sides and an upright centre line, but no flat (horizontal) line inside the square. The small diamond in the bottom half — between the bottom-middle point, the centre and the points where the diagonals cross the big diamond — is cut by the upright centre line, giving (c). (a) and (b) need a flat line through the small diamond, which X does not have, and (d) needs a flat line through the centre.