Final answer:
Adam can prove that quadrilateral PQRS inscribed in a circle is a square by showing that the diagonals are perpendicular, congruent, and they bisect each other, as per option D.
Step-by-step explanation:
To prove that quadrilateral PQRS is a square when inscribed in circle O, Adam must show that the diagonals of PQRS have specific properties that are characteristic of a square. Adam should demonstrate that the diagonals are perpendicular to each other, they are congruent (that is, they have equal length), and they bisect each other (each diagonal cuts the other into two equal parts). To satisfy all the conditions that define a square, option D is correct: Adam must show that the diagonals are perpendicular, congruent, and bisect each other.