Which of the following cannot be a cyclic quadrilateral?
Parallelogram
Trapezium
A quadrilateral is cyclic only when the sum of either pair of opposite angles is 180∘.
Each angle of a square and rectangle is 90∘. Hence both pairs of opposite angles are supplementary.
In parallelograms, the sum of adjacent angles is 180∘. Only in the case of rectangles and squares, the opposite angles can also be 180∘. For other parallelograms, this is impossible since the sum of one pair of opposite angles will be an obtuse angle and the sum of the other pair of opposite angles will be an acute angle.
A trapezium can also be a cyclic quadrilateral provided it is an isosceles trapezium in which the pair of non-parallel sides are equal.