Key Assumption of Multiple Regression Analysis

Consider a multiple regression model used to predict the sales of a company based on several independent variables, such as advertising spend, number of salespeople, and market size. For the model to provide valid inferences and predictions, certain assumptions must hold true. One of these assumptions is critical for the reliability of the coefficient estimates and is often checked through residual analysis.

Which of the following statements correctly describes one of the key assumptions of multiple regression analysis?