On the accompanying structural formula,
circle the acid functional group, only.

The -COOH attached to the CH3 is the acid functional group.
The term functional group refers to the atom, group of atoms or bond which accounts for the chemical reactivity of a compound.
The acid functional group is known by the formula -COOH. In this case, we can see that the -COOH attached to the CH3 is the acid functional group.
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