Choice A is the correct answer. Check for vertical alignment.

1. Formula: $m = \frac{-2 - 8}{5 - 5} = \frac{-10}{0}$.
2. Result: Division by zero is undefined.
3. Visual: The x-coordinates are the same, so it's a vertical line.

💡 Strategic Tip: If $x_1 = x_2$, the slope is undefined.

Choice B is incorrect because 0 slope is for horizontal lines. Choice C is incorrect.Choice D is incorrect.