First Principles Thinking: what changes in a slope test
B is correct. Both statements are true. In simple linear regression, slope tests use n minus 2 degrees of freedom because two parameters, the intercept and slope, are estimated. That does not change when the hypothesized slope is 0 or 1.0. The reason is also true because the t-statistic compares the estimated slope with the hypothesized population slope and scales that difference by the standard error of the slope.
Why option A is incorrect: R does not explain why the degrees of freedom are unchanged. The unchanged degrees of freedom come from the number of estimated parameters, not from the subtraction step itself.
Why option D is incorrect: the assertion is not false; the governing rule uses the same n minus 2 degrees of freedom for such slope tests.