The primary tool for determining whether the assumptions made about the regression model are appropriate is

A) ​interval estimation.
B) ​significance testing.
C) ​least squares regression.
D) ​residual analysis.

Residual analysis is the primary tool for determining whether the assumptions made about the regression model are appropriate. Residuals are the differences between the observed values and the values predicted by the model. Residual analysis checks if the residual plot has a random pattern, if the residuals have a constant variance, and if they are normally distributed. If these assumptions are violated, it indicates that the model may not be appropriate for the data. Therefore, option D is the correct answer.