One of the answer figures is hidden in figure X — same size and the same way up, though it may be anywhere in X. Which one?

Look at the top-right part of X: the middle line, the right side and the long diagonal make a right triangle with its right angle at the bottom-right, and the short slanting line from the top edge runs from the middle of that triangle's long side down to its right angle. That is exactly (a). (b) is its mirror image (the matching triangle in the top-left has no line to its corner), (c) needs an upright line from the middle of the long side, and (d) needs a second diagonal across the bottom-left square.