What Does It Mean to Say "A Between-Groups Effect Size

Multiple Choice

What does it mean to say "A between-groups effect size of .6 was achieved for the treatment group as compared to the no-treatment group"?

A) The group getting the treatment was .6 standard deviations higher on the outcome.
B) The group getting the treatment was .3 standard deviations higher on the outcome.
C) Both groups improved, but the no-treatment group showed a greater increase in scores.
D) All of the above.