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

Phenol contains an -OH group directly attached to an aromatic ring, so the lone pair on oxygen is in conjugation with the ring. Such conjugated -OH groups form coloured coordination complexes with $$Fe^{3+}$$ ions.
Reaction: $$3\,C_6H_5OH + FeCl_3 \;\longrightarrow\; \left[C_6H_5O\right]_3Fe + 3\,HCl$$
The complex $$\left[C_6H_5O\right]_3Fe$$ imparts a characteristic violet (purple/blue) colour to the solution.

Benzoic acid, $$C_6H_5COOH$$, does not have a phenolic -OH group. When treated with neutral $$FeCl_3$$ it either shows no colour or gives only a buff-coloured precipitate of ferric benzoate; no violet coloration appears. Hence phenol and benzoic acid can be distinguished by the colour test with neutral $$FeCl_3$$.

Checking the other options:

• Tollen’s reagent: used for aldehydes, gives no characteristic difference for phenol or benzoic acid.
• Molisch reagent: test for carbohydrates, not applicable here.
• Aqueous $$NaOH$$: both phenol (weak acid) and benzoic acid (stronger acid) dissolve forming sodium phenoxide and sodium benzoate respectively, so it cannot distinguish them.

Therefore, the suitable reagent is neutral $$FeCl_3$$.

Option C which is: Neutral $$\text{FeCl}_3$$