Lines, angles, and triangles SAT question

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 $(180-23-66{)}^{\circ }$, or $91^{\circ }$.

Choice A is incorrect and may result from conceptual or calculation errors.

Choice B is incorrect. This is the sum of the measures of $\angle X$ and $\angle Y$, not the measure of $\angle Z$.

Choice D is incorrect and may result from conceptual or calculation errors.