DSE 2018 Question 15: Linear regression and OLS

In a multiple regression model with three right-hand-side variables and $105$ observations estimated by OLS, the researcher must decide whether to include a fourth right-hand-side variable. The residual sum of squares is $250$ with four variables and $300$ with three variables. Critical values of the $F$-table ($\alpha = 0.05$) are $F(1,100) = 3.89$, $F(2,100) = 3.09$, $F(3,100) = 2.70$, $F(4,100) = 2.46$, $F(5,100) = 2.31$. This means when the fourth variable is included, the fit of the regression