The correct answer is B. 0.80.
The coefficient of determination is a measure of how well the data fit a straight line. It is calculated by squaring the correlation coefficient. So, if the coefficient of determination is 0.64, then the correlation coefficient is $\sqrt{0.64} = 0.8$.
The correlation coefficient is a measure of the strength of the linear relationship between two variables. It is a number between -1 and 1, inclusive. A correlation coefficient of 0 indicates no linear relationship between the variables. A correlation coefficient of 1 indicates a perfect positive linear relationship, and a correlation coefficient of -1 indicates a perfect negative linear relationship.
In this case, the coefficient of determination is 0.64, which indicates that the data fit a straight line very well. This means that the correlation coefficient is also very close to 1, and the most likely value is 0.8.