Context: $m \parallel n$. $x = 6k+13$, $y = 8k-29$. Alternate interior: $x = y$.

Answer: $6k+13 = 8k-29 \Rightarrow k = 21$. $x = 139$. $z = 180-139 = 41$.

Why C: Alternate interior angles are equal; $z$ is supplementary to $x$.

Traps: A ($3$) wrong setup. B ($21$) value of $k$, not $z$. D ($139$) value of $x$, not $z$.