Figure ABCD is plotted on a coordinate plane. The figure transforms to create figure A'B'C'D'. Which transformation took place?

Answer
Reflection on the y-axis.

Explanation. 
The diagram was not provided but I was able to get in the net.
It could seen that every point correspond to its image directly horizontally across the y-axis. All point are equidistant from their objects. It's like a reflection with y-axis as the mirror line. 
This is a reflection along the y-axis.