Question

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

Explanation:

Step1: Recall linear pair definition

A linear pair consists of two adjacent angles that form a straight line, so their sum is $180^\circ$, share a common vertex and side, and their non-common sides are opposite rays.

Step2: Analyze first diagram

Angles 1 and 2 are vertical angles (opposite each other at intersecting lines), not adjacent or forming a straight line.

Step3: Analyze second diagram

Angles 1 and 2 share a common vertex and side, their non-common sides form a straight vertical line, so they are adjacent and their sum is $180^\circ$.

Answer:

The second diagram (with the right angle marker, where angles 1 and 2 share a side and form a straight line)