Answer: c) 81
Explanation:
The formula to calculate the degrees of freedom error for a one-way within-subjects ANOVA is given by :-

Given : A researcher recorded the shooting percentage of 28 professional basketball players in each of four quarters in a game.
Here , n= 28 and k= 4
Then, the degrees of freedom error for a one-way within-subjects ANOVA will be :-

Hence, the degrees of freedom error for a one-way within-subjects ANOVA are 81.