Answer: Perpendicular.
Explanation:
Suppose that you have two perpendicular lines:
Remember that a line is something like:
y = a*x +b
and two lines are parallel if they have the same slope (a) but a different y-intercept(b)
Then our lines can be:
y1 = a*x + b1
y2 = a*x + b2.
Now, if we have a line:
y = a*x + b
Then a perpendicular line will have a slope equal to -(1/a):
yp = (-1/a)*x + c
So this only depends on the slope, and we know that our two parallel lines have the same slope. So if we construct a transversal line that is perpendicular to one of our lines, it also must be perpendicular to the other line.