Apoint has the coordinates (m, 0) and m ≠ 0.

which reflection of the point will produce an image located at (0, –m)?

a reflection of the point across the x-axis
a reflection of the point across the y-axis
a reflection of the point across the line y = x
a reflection of the point across the line y = –x

When you reflect a point across the line y = -x, the x-coordinate and the y-coordinate change places and are negated (the signs are changed).
For example : The reflection of the point (x, y) across the line y = -x  is the point (-y, -x)
 So the best option is : a reflection of the point across the line y = –x