The correct answer is C. relevant range.
A relevant range is the range of activity within which assumptions about cost behavior are valid. It is the range of activity over which the cost is expected to change in a predictable manner.
A vertical dashed line in a graphical representation of a cost function represents the relevant range. This is because the cost function is only valid within the relevant range. Outside of the relevant range, the cost function may not be accurate.
Option A is incorrect because the vertical dashed line does not represent the cost representation. The cost representation is the entire graph, not just the vertical dashed line.
Option B is incorrect because the vertical dashed line does not represent the irrelevant range. The irrelevant range is the range of activity outside of which the cost is not relevant to decision-making.
Option D is incorrect because the vertical dashed line does not represent the graphical representation. The graphical representation is the entire graph, not just the vertical dashed line.