Which of the following reagents may be used to distinguish between phenol and benzoic acid?

Phenol reacts with neutral ${\text{FeCl}}_3$ to give a violet-coloured complex. Benzoic acid reacts with neutral ${\text{FeCl}}_3$ to give a buff-coloured precipitate of ferric benzoate. Thus, ${\text{FeCl}}_3$ can distinguish them.