Final answer:
Phoebe's riddle is describing a trapezoid, a quadrilateral with one pair of parallel sides and two congruent edges (the non-parallel sides).
Step-by-step explanation:
Phoebe's riddle describes a quadrilateral with two congruent edges and exactly one set of parallel sides. The shape that fits this description is a trapezoid (also known as a trapezium in some countries). A trapezoid is defined as a four-sided figure, or quadrilateral, with at least one pair of parallel sides. The congruent edges referred to in the riddle could be the non-parallel sides of the trapezoid. This shape is unique among quadrilaterals in having exactly one pair of parallel sides, differentiating it from shapes like rectangles and squares, which have two pairs of parallel sides, and from shapes like parallelograms, which also have two pairs of parallel sides but in equal length.