The correct answer is: D. Varies between ± 1.
The coefficient of correlation is a statistical measure that indicates the strength and direction of the linear relationship between two variables. It is denoted by the letter $r$ and can range from -1 to 1. A value of $r=0$ indicates that there is no linear relationship between the two variables. A value of $r=1$ indicates a perfect positive linear relationship, meaning that as one variable increases, the other variable also increases in a perfectly straight line. A value of $r=-1$ indicates a perfect negative linear relationship, meaning that as one variable increases, the other variable decreases in a perfectly straight line.
The coefficient of correlation is always between -1 and 1, inclusive. This is because the two variables can always be perfectly correlated, perfectly negatively correlated, or not correlated at all.