Final answer:
The slope of a line in the regression equation represents the relationship between the independent variable and the dependent variable.
Step-by-step explanation:
The slope of a line in the regression equation represents the relationship between the independent variable and the dependent variable. In the example given, the regression equation represents the relationship between years worked and annual salary. The slope tells us how the annual salary changes for every one unit increase in years worked, on average. So the correct answer is d. The relationship between years worked and annual salary.