Answer: first option.
Explanation:
Knowing that the transformation T maps triangle XYZ onto triangle X'Y'Z', you can identify from the figure that the triangle XYZ is the preimage, because it is the original shape and the triangle X'Y'Z' is the image, because is the shape obtained after the transformation.
You can observe in the figure that the image of the vertex X is the vertex X', the image of the vertex Y is the vertex Y' and the image of the vertex Z is the vertex Z'.
Therefore, the conclusion is: The image of Y is Y'.