Answer:
The correct answer is C. 129.
Explanation:
Let's recall that the Inscribed Quadrilateral Theorem states that a quadrilateral is inscribed in a circle if and only if the opposite angles are supplementary.
In the case of the inscribed quadrilateral ABCD in the graph attached, we have that:
m∠C= 51°, therefore its supplementary angle, ∠A, should be the difference between 180° and m∠C.
m∠C= 180° - m∠A
Replacing with the real values:
51 = 180 - m∠A
m∠A = 129°
The correct answer is C. 129.