The correct answer is C. correlation coefficient.
A correlation coefficient is a statistical measure that shows the strength and direction of the linear relationship between two variables. It is a number between -1 and 1, where -1 indicates a perfect negative correlation, 0 indicates no correlation, and 1 indicates a perfect positive correlation.
A tendency coefficient is a measure of the tendency of a variable to vary. It is a number between 0 and 1, where 0 indicates no tendency to vary and 1 indicates a perfect tendency to vary.
A variable coefficient is a measure of the variability of a variable. It is a number between 0 and 1, where 0 indicates no variability and 1 indicates a perfect variability.
A double coefficient is a measure of the correlation between two variables. It is a number between -1 and 1, where -1 indicates a perfect negative correlation, 0 indicates no correlation, and 1 indicates a perfect positive correlation.
In conclusion, the correct answer is C. correlation coefficient.