Final answer:
If the best estimate for Y is the mean of Y, then the correlation between X and Y is zero.
Step-by-step explanation:
The correlation between X and Y can be positive, negative, or zero, depending on the relationship between the two variables. The correlation coefficient measures the strength and direction of the linear association between X and Y. If the best estimate for Y is the mean of Y, it means that there is no linear relationship between X and Y, and the correlation between X and Y would be zero.