Choice A is the correct answer. Undefined slope means vertical.

1. Form: Vertical lines are $x = k$.
2. Value: The x-coordinate is $-3$.
3. Equation: $x = -3$.

💡 Strategic Tip: Vertical lines lock the x-value. Horizontal lines lock the y-value.

Choice B is incorrect because it is horizontal. Choice C is incorrect because it has a slope of -3. Choice D is incorrect because it uses the y-value.