The quadrant that the reflect image is in is quadrant IV. This means the y coordinate is negative but the x stays the same.
Following, when you reflect any point or shape across the axis, the actual place of the point doesn't change (ie. (2, 1) reflected over x axis --> (2, -1))
This means the actual numbers in the coordinate stays the same, but the y values are negative.
Which means B and C are immediately eliminated. Finally, point C is (2 , 1), however answer D says that point C' is (1, -2) which doesn't match up since it should be (2, -1). Therefore your answer is A.