Which figure represents the image of parallelogram LMNP after a reflection across the line y = x?

When you reflect a point across the line y = x, the x-coordinate and y-coordinate change places. This gives you such reflection rule:

From the diagram:
L(3,1), M(4,3), N(5,3) and P(4,1).
Using the reflection rule, you can find coordinates of image points:
L'(1,3), M'(3,4), N'(3,5) and P'(1,4).
As you can see, these are coordinates of vertices of the figure C.
correct choice is option 3 - figure C.