The coefficient of determination is the percentage of variance in one variable that ______.

A) is accounted for by the variance in the other variable
B) matters only when the other variable is constant
C) equals the square root of the Pearson's product-moment correlation coefficient
D) can be determined by taking the square root of the phi coefficient

The coefficient of determination, also known as R-squared, represents the proportion of variance in one variable that can be explained by the variance in the other variable. Therefore, option A is the correct choice. Option B is incorrect because the coefficient of determination is relevant regardless of whether the other variable is constant or not. Option C and D are also incorrect because neither the square root of the Pearson's product-moment correlation coefficient nor the phi coefficient can directly determine the coefficient of determination.