The angle formed when a straight angle is bisected is called _________.

The correct option is D a right angle
The angle in a straight line is ${180}^{\circ }$.
An angle bisector divides an angle into two halves.
$\Rightarrow \frac{180}{2}={90}^{\circ }$, which is a right angle.
Hence, $\angle ACD=\angle DCB={90}^{\circ }\text{(CD is the angle bisector)}$