Final answer:
The outer sides of two adjacent supplementary angles are parallel because they form a straight line when extended, which means they do not intersect at any point.
Step-by-step explanation:
If two adjacent angles are supplementary, this means that the sum of their measures is 180°. The outer sides of these angles form a straight line when extended. Thus, they are parallel lines because parallel lines are two lines in a plane that do not intersect or touch at any point, exactly like a straight line.
The outer sides cannot be perpendicular, since perpendicular lines intersect at a 90° angle. They also cannot be congruent, as this term applies to figures of the same shape and size, which doesn't necessarily relate to the orientation of lines.
Therefore, the correct answer is A) Parallel.