See the picture attached to better understand the problem
we know that A quadrilateral is called Cyclic quadrilateral <span>if its all vertices lie on the circle. </span><span>The opposite angles of a Cyclic - quadrilateral are supplementary </span>so ∠A+∠C=180° and ∠B+∠D=180°
therefore the answer is <span>C) The sum measure of ∠B and ∠D equals 180°</span>