Which transformation is not always congruent with its original image? a) dilation b) reflection c) rotation d) translation

A) dilation because it changes the size of the shape 

The answer is A, Dilation.

Congruent images mean the 2 (or more) images/shapes are completely identical to each other, down to their length of sides or their angles. 

Dilation means the enlargement of images since the whole image is enlarged, the length or area must be different, so we can already decide A is the answer. 

On the other hand, reflection is where the image is for example shown in the other way round, depending on where the reflection line is. The image still has the same dimensions, it's just flipped over.

Rotation is where the image is turned and may be presented in a different direction, depending on the degree of rotation. Again, the image still has the same dimensions, just that it faces another direction.

And lastly, translation is where the image is relocated. Also, size and dimension are not changed, just that the image sits in another place.

Therefore, your answer is A.