Point C is reflected across a line AB such that it maps to point D.
Points C and D are then connected to form a line CD.
Let us draw a figure to better understand the problem
As you can see from the above figure,
We have reflected the point C across the line AB and the reflected point is mapped to point D.
Then we connected the point C to point D using the red color to form line CD.
The line CD is perpendicular to line AB.
All other options depend on the exact scenario which means they may or may not be true.
But we can be sure about option D that it must always be true.
Therefore, option D is the correct answer.