Answer: a. reflecting
Step-by-step explanation:
The rigid transformations are the transformation which transform any figure to exactly congruent image because it preserves the side length and the angles of a figure.
Reflection is a rigid transformation that flips a figure to create a new figure. It is called so because it always produces the image which is the same size and shape as the pre-image .
Hence, reflection is a congruence transformation.
Stretching and dilation both changes the size of the actual figure therfore they do not produce congruent images.
