Which statistic is an estimate of the magnitude of the difference between the means of two groups measured in standard deviation units?

A) r²
B) t²
C) Cohen's d
D) β

Cohen's d is a commonly used effect size measure that estimates the magnitude of the difference between the means of two groups in standard deviation units. It is calculated by taking the difference between the means of the two groups and dividing it by the pooled standard deviation of the groups. This allows for the comparison of effect sizes across different studies and variables.