Final answer:
The length of side BC remains 5 units after the quadrilateral ABCD is translated to form quadrilateral AB, as translations do not change the dimensions of a figure.
Step-by-step explanation:
The question asks for the length of side BC in a translated quadrilateral ABCD which forms quadrilateral AB, with given side lengths for AB, BC, CD, and AD. Since translations do not change the size or shape of a figure, just its position, the side lengths of the quadrilateral remain unchanged. Therefore, if the original quadrilateral had side BC measuring 5 units, after the translation, the side BC of quadrilateral AB will still measure 5 units.