Answer:
Parallel
Explanation:
The complete statement would be "if a line is perpendicular to one of two parallel lines, then it is also perpendicular to the other".
The reason for this is because parallel lines have the same slope, that's the condition. On the other hand, parallel lines have opposite and reciprocal slopes.
So, if you think this through, if a line is perpendicular to another, then it's going to be perpendicular to all different lines which are parallel to the first one, because all their slopes are equivalent, and they will fulfill the perpendicularity condition.