The first transformation of the rectangle ABCD to rectangle A'B'C'D' is a reflection over the x-axis.
You can determine this because the image is equidistant from the x-axis as the preimage but is inverted as if it was reflected in a mirror.
Next is the transformation from A'B'C'D' to A''B''C''D'', as you can see the first figure is lying over it's longest side and after the transformation it is lying over its shortest side, this indicates that the figure was rotated.
So the rectangle ABCD was reflected over the x-axis to A'B'C'D' and then rotated counterclockwise to A''B''C''D''
The correct choise is A. a reflection followed by a rotation.