The correct answer is: by its slope.
In a simple linear regression model, the output variable will change by the slope of the line for every unit change in the input variable. The slope is the coefficient of the independent variable in the regression equation. It tells us how much the dependent variable changes for every unit change in the independent variable.
For example, if the slope is 2, then for every unit increase in the independent variable, the dependent variable will increase by 2 units.
The intercept is the value of the dependent variable when the independent variable is 0. It is the point where the line crosses the y-axis. The intercept is not relevant to the question of how much the output variable will change for every unit change in the input variable.
Therefore, the correct answer is: by its slope.