SAT Question f9d40000: Lines, Angles, and Triangles answer and explanation with Desmos solution

Choice C is correct. The triangle angle sum theorem states that the sum of the measures of the interior angles of a triangle is $180^{\circ }$. It's given that in $△XYZ$, the measure of $\angle X$ is $23^{\circ }$ and the measure of $\angle Y$ is $66^{\circ }$. It follows that the measure of $\angle Z$ is ${\left(180-23-66\right)}^{\circ }$, or $91^{\circ }$.