Step 1: Define reference group.
A reference group is a social group that individuals use as a standard for evaluating their own behavior, attitudes, and values.
Step 2: Function.
It guides behavior, sets norms, and helps in self-assessment and goal-setting, even if the individual is not a member of the group.
Step 3: Conclude.
\[
\boxed{\text{Reference group (Option A) is used for comparison.}}
\]