Final answer:
△def is congruent to △d'e'f' because both a translation and a reflection are rigid motions that preserve size and shape.
Step-by-step explanation:
The question you're asking deals with the transformations of a triangle in the coordinate plane and whether these transformations preserve congruence. When the rule (x, y) → (x, y + 1) is applied, it represents a translation one unit up along the y-axis. The next rule applied, (x, y) → (x, -y), reflects the triangle across the x-axis.
Since both a translation and a reflection are rigid motions (movements that preserve distance and angles), the size and shape of the triangle are not changed. Therefore, △def is congruent to △d'e'f' because the rules represent a sequence of rigid motions.