Only a horizontal line has zero slope. A horizontal line's equation is given in the form: "y equals a number" So y=-3 is horizontal with zero slope.
Vertical lines have undefined slope. This is not the same as zero. Vertical lines have an equation in the form x= anumber. Above in the answer choices, choice a and d are both vertical with undefined slope and are not the correct answer. Choice c has slope 5, because that is the number beside the x in the form y=mx+b where m is the slope. So choice c is also not the answer.
