For the cyclic quadrilateral abcd, which statement is true? a) the sum measure of ∠a and ∠b equals 180°. b) the sum measure of ∠a and ∠d equals 180°. c) the sum measure of ∠b and ∠d equals 180°. d) the sum measure of ∠c and ∠d equals 180°.

See the picture attached to better understand the problem

we know that
A quadrilateral is called Cyclic quadrilateral if its all vertices lie on the circle.
The opposite angles of a Cyclic - quadrilateral are supplementary
so
∠A+∠C=180°
and
∠B+∠D=180°

therefore
the answer is
C)The sum measure of ∠B and ∠D equals 180°