Question

in which diagram do angles 1 and 2 form a linear pair?

Explanation:

A linear pair of angles are adjacent angles that share a common side, their non-common sides form a straight line, and their measures add up to $180^\circ$.

1. For the top diagram: Angles 1 and 2 are vertical angles (opposite each other at an intersection), not adjacent, so they do not form a linear pair.
2. For the bottom diagram: Angles 1 and 2 are adjacent, share a common vertex and side, and their non-common sides form a straight line (the horizontal and vertical lines meet at a right angle, and the two angles occupy the space along that straight angle), so they form a linear pair.

Answer:

The bottom diagram (the second option with the right angle and two internal angles labeled 1 and 2)