The translation that maps triangle ABC to A prime B prime C prime would be a reflection across the y axis. This is because when you reflect something, you are pretty much flipping it. When you reflect across the y axis, you are flipping the triangle across the y axis. Take one point for example. I will use C. Notice how the point C is 3 units away from the y axis. So the same way you would move the point 3 units right from the y axis, and that would be your new point. This sounds kind of complicated, so I will give you a list of rules to make it more simple.
Reflection across y axis: (x,y) would be equal to (-x, y)
Reflection across x axis: (x,y) would be equal to (x, -y)
Reflection across y = x: (x,y) would be equal to (y,x)
Reflection across y = x: (x,y) would be equal to (-y,-x).
A reflection across y = x would be when you have a line that for every 1 it rises, it goes right 1. It is a positive line, as opposed to the y = -x line. It also has a slope of 1. I will try attaching a graph if I can.
Anyway, as I was saying. So pretty much if you don't want to go through the logic, to see whether a figure is reflected, just try each of these rules and if one works then you have your answer. Otherwise it would not be a reflection.
Thanks for being a great mod and hope this helps! :D