1. The problem asks: If a line is vertical, what is its slope? 2. Recall the definition of slope: slope $m = \frac{\Delta y}{\Delta x}$. 3. For a vertical line, the change in $x$ is zero ($\Delta x = 0$), which means the slope formula involves division by zero. 4. Division by zero is undefined in mathematics. 5. Therefore, the slope of a vertical line is undefined. Final answer: D. its slope is undefined