Yes, you are correct in thinking that the negative reciprocal of 5/1 is -1/5. However, in this case, the given equation y = 5 represents a horizontal line. Since the line is horizontal and parallel to the x-axis, any line perpendicular to it would be a vertical line parallel to the y-axis.
The equation of any vertical line can be written as x = constant, where the constant represents the x-coordinate of the points on the line. Therefore, the correct choice for the equation of a line perpendicular to y = 5 would be x = constant.
For example, x = 3, x = -2, x = 7, etc., would all be equations of lines perpendicular to y = 5.