The line y=1 is the horizontal line passing through the point (0, 1). We have drawn it with red.
The reflection across this line of each point X, Y, and Z is such that the distance of each X, Y, and Z is equal to the distance of each X', Y', and Z', respectively, to the red line.
According to our drawing, we see that the coordinates of X' are (3, 3).
Answer: a.