Final answer:
The rule that describes a reflection across the line y = x is Tx = (y, -x).
Step-by-step explanation:
To find the rule that describes a reflection across the line y = x, we need to swap the x-coordinate with the y-coordinate for each point. In this case, the points are D(2, -4), E(1, -1), and F(5, 1).
Swapping the coordinates, we get D'(-4, 2), E'(-1, 1), and F'(1, 5). So the rule for reflection across the line y = x is Tx = (y, x), which corresponds to option b).