Answer:
The last one
Explanation:
The fast reason why - all the other scenarios require 3 lines: 2 lines both parallel to a 3rd, or 2 lines cut by a transversal line, etc. The diagram only shows 2 lines. So the right answer must be the one that doesn't require a third line to be present. :)
The longer answer why - See how angles 2 & 3 have the same style of arc? That is telling you they are of equal measure. So together they are a straight angle, 180°. And they are equal to each other, so 2x = 180° --> each one must be a 90° angle.
Wherever two lines cross, if one angle is a right angle, all 4 angles at the intersection also must be right angles, so everything is perpendicular.