Given that quadrilaterals ABCD, LMNO, and STUV are congruent (≅), and STUV is a rectangle with diagonal SU = 5 cm, we can use the properties of congruent figures.
In a rectangle, the diagonals are equal in length. Therefore, diagonal SV is also 5 cm in quadrilateral STUV.
Since ABCD ≅ STUV, the diagonals AC and BD of quadrilateral ABCD will also be equal to the diagonals of STUV.
Thus, according to the Transitive Property of congruence, we have:
AC = SU = 5 cm BD = SU = 5 cm
Therefore, the lengths of diagonals AC and BD are both 5 cm.
So the answer is: AC and BD = 5 cm.