Hello there. To solve this question, we'll have to find which segments from the parallelepiped are skew to the line BF.
First, remember skew lines are those that doesn't intersect each other, usually they have different orientations and belongs to opposite faces.
For instance, take a cube as an example:
I'll color some lines in blue and those skew to the blues will be in yellow.
Now, we'll draw the same parallelepiped and see which segments will be skew to BF:
Coloring BF in red, will color the skew lines to it in green.
As you can see, AB, EF, BC and FG are not skew because they're perpendicular to BF and are in the same plane containing that segment.
CD, GH, HE, DA are skew to it.
Neither of the four lines are in the same plane from BF, therefore we've met the definition.