Answer: Reflection about x-axis
Step-by-step explanation:
Given
Initially, vertices are [tex]A(-2,0), B(2,5),C(3,1)[/tex]
After reflection they become [tex]A(-2,0),B(2,-5),C(3,-1)[/tex]
It is clear that the x coordinate remains unchanged and the y coordinate changes its sign. It occurs when reflection is done about the x-axis.
For example, when (a,b) is reflected about the x-axis, it becomes (a,-b).
So, here reflection is being done about the x-axis.