Question

what does it mean for triangles △abc and △def to be congruent?
all sides are parallel
all corresponding sides and angles are equal.
all angles are right angles.
the triangles have the same perimeter.

Explanation:

Congruent triangles have all corresponding sides and angles equal. Parallel sides (first option) don't define congruence. All right - angled angles (third option) is a property of right - angled triangles, not congruent ones. Same perimeter (fourth option) doesn't guarantee congruence as side lengths could differ.

Answer:

All corresponding sides and angles are equal.