B is the answer (PLATO)
B. m∠A + m∠C = m∠B + m∠D
To inscribe a quadrilateral in a circle, the sums of its opposite angles must be equal (each sum of its opposite angles equals 180 °)
Opposite angles of an inscribed quadrilateral are supplementary, so it is true that ...
... B. m∠A + m∠C = m∠B + m∠D . . . . . = 180°