A parallelogram inscribed in a circle is

Parallelogram inscribed in a circle

For the parallelogram opposite angles are equal

For cyclic quadrilateral sum of opposite angles is 180°

Let the angle be $x°$

$\therefore x+x=180°\Rightarrow 2x=180°\Rightarrow x=90°$

A parallelogram with one of the angle 90° is rectangle