The correct answer is A. straight line.
A straight line is a line that has no curves. It is the shortest distance between two points. In statistics, a straight line is used to represent a linear relationship between two variables. A linear relationship is a relationship in which the change in one variable is proportional to the change in the other variable.
A curved line is a line that has at least one curve. It is longer than a straight line between the same two points. In statistics, a curved line is used to represent a nonlinear relationship between two variables. A nonlinear relationship is a relationship in which the change in one variable is not proportional to the change in the other variable.
A horizontal line is a line that runs parallel to the x-axis. It has a constant y-value. In statistics, a horizontal line is used to represent a constant value.
A vertical line is a line that runs parallel to the y-axis. It has a constant x-value. In statistics, a vertical line is used to represent a point.
In the question, the line is used to join observations with lower and highest values of cost driver. This means that the line must pass through the lowest and highest points on the graph. The only line that can do this is a straight line.