Choice A is the correct answer. Vertical lines have an undefined slope.

1. Identify: The equation $x = 5$ represents a vertical line.
2. Reasoning: Vertical lines have a "run" of 0. Since slope is $\frac{\text{rise}}{\text{run}}$, dividing by zero is undefined.

💡 Strategic Tip: Think of skiing: A horizontal floor has 0 slope (flat). A vertical cliff has undefined slope (you fall).

Choice B is incorrect because a slope of 0 belongs to horizontal lines ($y=k$). Choice C is incorrect because 5 is the x-intercept, not the slope. Choice D is incorrect because a slope of 1 is a diagonal line ($y=x$).