For this question malabrec31 is correct. Although, I saw confusion in the comments.
This question may be slightly different for other people.
If we took this question but had counterclockwise instead of clockwise, the answer would be altered. We would move from (x,y) to (-x,y).
I have attached the graph.
We can see the slope of BA is 1, this is 2/2 following rise/run. If we then rotate the figure 90 degrees counterclockwise segment B'A' would have a slope of 2/-2, this is the same as -1.
Now we can take out answer choices B and C.
Next, we need to figure out if the new segment is parallel or perpendicular to the original segment.
If you imagine what the two figures look like in the graph attached, you can see they would not be parallel.
If we rotated counterclockwise instead of clockwise the answer would be A, "Side B'A' has a slope of -1 and is perpendicular to side BA."
This is not the answer to the given question but instead to clarify for the viewers. Thank you.