Choice A is the correct answer. Definition of axes.

1. X-axis: All points on this line have a height ($y$) of 0.
2. Equation: $y = 0$.

💡 Strategic Tip: The X-axis is $y=0$. The Y-axis is $x=0$. It's counter-intuitive!

Choice B is incorrect because it is the y-axis. Choice C is incorrect because it is a diagonal line. Choice D is incorrect because it is a vertical line at 1.