Remember that SAS stands for side-angle-side, meaning two sides and the angle formed by those sides must all be congruent for two triangles to be congruent.
In this case, you have a side—AB and XY—and an angle, <B and <Y. To complete this, you simply need the final side, which forms angle B along with side AB on triangle ABC, and forms angle Y along with side XY on triangle XYZ.
By looking at the diagram, you can tell this side on triangle ABC is BC, and the side on triangle XYZ is YZ, so the answer must be B.
