Question

if two triangles are congruent, which transformation could map one onto the other?
stretching
rotation
changing side lengths
changing angle measures

Explanation:

Congruent triangles have the same shape and size. Rotation is a rigid transformation (a transformation that preserves shape and size). Stretching changes the size (so side - lengths change), changing side - lengths directly alters the size, and changing angle measures changes the shape. Rigid transformations (like rotation, translation, reflection) can map congruent triangles onto each other.

Answer:

Rotation