"Line L has an undefined slope" so line L is any vertical line.
Anything perpendicular to this vertical line will be horizontal. So line M is horizontal.
Horizontal lines are of the form y = k for some fixed number k. The only thing that fits is choice B) y = 7 where k = 7 in this case.
Choice A graphs out a diagonal line, choice C graphs a vertical line, while choice D graphs out a hyperbola. These facts are why we can eliminate these non-answers.
