Final answer:
The vertical line is the only straight line that fails the vertical line test because it intersects with a graph at multiple points for a single x-value, suggesting it is not a graph of a function.
Step-by-step explanation:
The only straight line that fails the vertical line test is a vertical line. The vertical line test is used to determine if a curve is a graph of a function or not. A function can only have one output (y-value) for each input (x-value). A vertical line would intersect with a graph at multiple points for a single x-value, implying multiple y-values for that input, which contradicts the definition of a function. Therefore, the correct answer to the question is b) Vertical line.