Introduction:
The angle between the regression lines is a measure of the relationship between two variables. It helps in understanding how the two variables are related to each other and how they change together. The angle between the regression lines can provide valuable insights into the strength and direction of the relationship between the variables.

Explanation:
The angle between the regression lines is determined by the correlation coefficient between the variables. The correlation coefficient measures the strength and direction of the linear relationship between the two variables. It ranges from -1 to +1, where -1 indicates a perfect negative relationship, +1 indicates a perfect positive relationship, and 0 indicates no relationship.

When the correlation coefficient is positive (greater than 0), the regression lines will have a positive angle between them. This means that as one variable increases, the other variable also tends to increase.

When the correlation coefficient is negative (less than 0), the regression lines will have a negative angle between them. This means that as one variable increases, the other variable tends to decrease.

When the correlation coefficient is zero, the regression lines will be parallel to each other, and the angle between them will be zero degrees. This indicates that there is no linear relationship between the variables.

Conclusion:
In conclusion, the angle between the regression lines depends on the correlation coefficient between the variables. The correlation coefficient measures the strength and direction of the linear relationship, and this information is reflected in the angle between the regression lines. Therefore, the correct answer is option 'A' - correlation coefficient.