The diagram where angles 1 and 2 form a linear pair is option D which is that a horizontal line has 1 line extending from it. Angles 1 and 2 are formed.
Given that angle 1 and 2 form a linear pair.
We know that a linear pair is an angle that are formed when two lines intersect each other at a single point.
In our case when a horizontal line has 1 line extending from it and when angles 1 and 2 are formed shows a linear pair.
In first part when two lines intersect they donot form 4 angles.
In second part when 3 lines extend from a point they don't form right angle between the lines.
Hence the linear pair angles are shown by the statement that a horizontal line has 1 line extending from it. Angles 1 and 2 are formed.
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