Answer
$C$
Work Step by Step
The definition of bisecting is that something is cut in two equal portions. If $\overline {BD}$ bisects $\angle ABC$, then $\overline {BD}$ cuts $\angle ABC$ into exactly two congruent angles.
In the answer options, only option $C$ shows that $\angle ABC$ is cut into exactly two congruent angles: $\angle ABD$ and $\angle CBD$. These angles are marked as being congruent in the diagram.
