Answer:
D. reflection across a vertical line
Explanation:
Reflection is basically a mirror image. Since you can see that the first triangle is flipped over to get the second one, this is a reflection.
It is not a vertical translation; this means that the triangle shifted up or down.
It is not a horizontal translation, this means that the triangle shifted left or right.
It is not a rotation about point A either, this would have meant that the triangle spun around.