Let's focus on one point, say point L = (-4,2)
It reflects over the horizontal x axis to land on point L ' = (-4,-2)
All that has changed is the y coordinate has flipped from positive to negative.
So the rule used is (x,y) ---> (x,-y)
Put this all together and the final answer is choice C
Note: the answer is not choice D as we're not reflecting over the y axis
