Answer
$d$
Work Step by Step
If three parallel lines cut through two transversals, then the segments on one transversal cut by the parallel lines are proportional to the segments on the other transversal cut by the parallel lines.
Now we can set up the proportions to compare the segments on one transversal to the segments on the other transversal:
$\frac{a}{b + c} = \frac{d}{e + f}$
