Answer:
C. straight
Explanation:
A Linear Pair is two adjacent angles whose non-common sides form opposite rays.
If two angles form a linear pair, the angles are supplementary.
A linear pair forms a straight angle which contains 180º, so you have 2 angles whose measures add to 180, which means they are supplementary.
In the figure given in attachment, AB and BC are two non common sides of ∠ABD and ∠DBC.
∠1 and ∠2 form a linear pair.
The line through points A, B and C is a straight line.
∠1 and ∠2 are supplementary.
Thus two non-common sides of adjacent supplementary angles form a straight angle.