The reflection shown in the picture is an example of what's called a rigid transformation, one that doesn't deform the original shape in any way. Rigid transformations preserve lengths and angles, which means that the measure of some angle ∠A is going to be equivalent to the measure of the corresponding transformed angle ∠A'
Here, we see that the only response which represents that correspondence is the last one: m∠BCA = m∠B'C'A'
