1. The problem states that quadrilateral $ABCD$ has one pair of opposite sides $AB$ and $CD$ that are both congruent and parallel. 2. Alessandra wants to prove that $ABCD$ is a parallelogram by showing congruence of a single pair of triangles. 3. The diagonals $AC$ and $BD$ intersect at point $E$, creating triangles $ABE$ and $CDE$. 4. Since $AB \parallel CD$ and $AB = CD$, and $E$ is the intersection of diagonals, triangles $ABE$ and $CDE$ share angle $AEB$ and $CED$ which are vertical angles and thus congruent. 5. Also, $AE = CE$ and $BE = DE$ because diagonals of a parallelogram bisect each other. 6. Using the Side-Angle-Side (SAS) criterion, triangles $ABE$ and $CDE$ are congruent. 7. Therefore, Alessandra is referring to triangles $ABE$ and $CDE$ and should use the side-angle-side criterion to establish congruence. **Final answer:** D