Answer: Choice (a)
Rotation 90 degrees counterclockwise about the origin.
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Explanation:
Plot a point at the origin. Let's call it point D.
Form segments AD and A'D.
Use the slope formula to note that those segments have negative reciprocal slopes, which leads to the segments being perpendicular.
This proves we have a 90 degree rotation. The direction is counterclockwise when going from A to A'.
The same idea applies when going from B to B', and also C to C'.