Final answer:
The correlation coefficient that allows perfect prediction of the Y score from the X score is r = -1, indicating perfect negative correlation.
Step-by-step explanation:
The value of r (correlation coefficient) tells us about the strength and direction of a linear relationship between two variables. For perfect prediction of one variable (Y score) from another (X score), we need a correlation coefficient that represents a perfect linear relationship. That would be when the value of r is exactly +1 or -1. The other options such as +2, which is beyond the possible range of r, or -0.5 which indicates a moderate negative linear relationship, or 0 which means no linear relationship at all, do not allow for perfect prediction. Thus, the correlation coefficient that allows perfect prediction is r = -1, which represents a perfect negative correlation, meaning that if one variable increases, the other variable decreases in a perfectly predictable manner.