Explanation:
congruent means that after any sequence of flipping and turning/rotating (but no size changing) you could put both shapes on top of each other, and they would cover each other perfectly - nothing is "hanging over" from the upper one, nothing is visible from the lower one.
a)
while A and B are correct, C could be turned 90° counterclockwise and be a perfect match for X. so, C is congruent too.
and D could be simply flipped about its left leg and be a prefect match for X.
C and D have the same side lengths as X (3 and 4 units) and the right angle between them. therefore, by Pythagoras, also the third side (the Hypotenuse) is equally long for X, C and D.
in short, Rowan has not used all possibilities of rotation and flipping.
b) all 4 triangles A, B, C and D are congruent to X.