Suppose we find a 0.8 correlation between students's reported interest in psychology and their grades on a psychology test. What conclusion can we draw?

The conclusion that could be made from this study suggests  that at a 0.8 correlation, the 2 variables (student interest in psychology and students grades in psychology)  show strong positive correlation. Hence if the interest in Psychology increases, there should be a corresponding increase in the students grades.

Explanation:

When a strong positive correlation is established, it shows that as one variable increases the other variable also increases. At 0.8 value of correlation, a strong positive correlation is established. Hence once the students interest in psychology increases there is also an increase in the grades in the subject or course.