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

Linear pairs form a straight angle (180 degrees) when they are put together.

Answer: angle1 and 2 in option b forms a linear pair.

Angle 1 and Angle 2 are a linear pair, if Angle 1 + Angle 2 = 180^o

Explanation:

Two angles are said to form a linear pair if they add up to 180 degrees. 

(a) 50^° + 40^° = 90^°

Since the sum of angles is not equal to 90^°, the angles 50^° and 40^° do not form a linear pair.

(b) 79^° + 101^° = 180^°

Since the sum of angles is equal to 180^°, the angles 79^° +and 101^° form a linear pair.

So, the angles in part (b) form a linear pair.