Answer:
B. reflection across the origin.
Explanation:
In this transformation, "reflection across the origin" wasn't applied, because the triangle is in the opposite side. Reflection across the origin is like having a mirror in the origin.
On the other hand, by rotation we get the transformation as it's shown, because the triangle would be in that exact spot. Similarly, if we reflect across the line y = -x, the triangle would be like that, because when we reflect across such line, all coordinates are changed by its opposite, for example C(1,3), changes to C'(-1,-3), which actually happens.
Therefore, the right answer is B.