Answer:
Dilation
Explanation:
There are two main broad types of transformations: rigid and non-rigid. Rigid transformations keep the general shape the same while changing its position, rotation, direction, etc. Non-rigid transformations, on the other hand, may change how big it is.
For the image to have a different area than the original triangle, it would have to be a different size. Therefore, the original triangle must have gone through a non-rigid transformation. The only non-rigid transformation is dilation, so that is the answer.
There is another type of non-rigid transformation called shearing; however, this is fairly uncommon. If you want, you can include that as a possibility in your answer as well or just put "non-rigid transformation."
Let me know if you want more clarification. :)