If two angles are supplementary, then their sum is
$
  {\text{A}}{\text{. 9}}{{\text{0}}^0} \\
  {\text{B}}{\text{. 18}}{{\text{0}}^0} \\
  {\text{C}}{\text{. 27}}{{\text{0}}^0} \\
  {\text{D}}{\text{. 36}}{{\text{0}}^0} \\
$

Hint – If two angles are supplementary then they always form a straight line, use this concept to reach the solution of the problem.

Let AB be a straight line.
And let P and Q are two angles.
As we know the condition of supplementary angle which is, two angle make a supplementary angle if the sum of two angles is ${180^0}$.
$ \Rightarrow \angle P + \angle Q = {180^0}$(on a straight line the sum of angle is ${180^0}$)
Hence, option (B) is correct.

Note – In such types of questions always recall the property of supplementary angle which is that the sum of two angles always make ${180^0}$, or we can also say that two right angle also make a supplementary angle, and we can also say that two supplementary angle always form a straight line.