If is a diameter, which of the following must be a semicircle?

There are two semicircles in the drawing: the arc going clockwise from a to c and the arc going counterclockwise from a to c.

AOB is adjacent to BOC

BOC is adjacent to COD

and

COD is adjacent to AOD

Since COD is 45 degrees, angle AOD is 180 degrees - 45 degrees or 135 degrees.

The circumference of the circle is pi * AC. The length of the arc AD = 135/360 * pi * AC = 1.18 * AC.