Answer:
D. 13
Explanation:
From the diagram,
and

In an isosceles trapezium, the base angles are equal.
This implies that

The side length CB of the trapezoid is a transversal line because CD is parallel to AB.
This means that
and
are co-interior angles.
Since co-interior angles are supplementary, we write and solve the following equation for
.
Group similar terms
Simplify both sides of the equation.

Divide both sides by 12


The correct answer is D.