Choice A is the correct answer. Vertical lines.

1. Form: Slope-intercept is $y = mx + b$.
2. Problem: Vertical lines have undefined slope ($m$) and cannot be written as $y = ...$.
3. Conclusion: Not possible.

💡 Strategic Tip: Vertical lines are the only lines that cannot be written in slope-intercept form.

Choice B is incorrect because it is horizontal. Choice C is incorrect because it is diagonal. Choice D is incorrect because it is horizontal.