Which reagent will distinguish Benzophenone from acetone?

The correct option is I₂/NaOH.

To understand why this reagent is used to distinguish benzophenone from acetone, let us analyze the structures and reactivity of both compounds:
1. Acetone has the chemical formula CH₃COCH₃. It contains a methyl ketone group, which is a carbonyl group ($-C=O$) directly attached to at least one methyl ($-C{H}_3$) group.
2. Benzophenone has the chemical formula C₆H₅COC₆H₅. It contains a carbonyl group bonded to two phenyl rings, meaning it lacks a methyl group directly attached to the carbonyl carbon.

The iodoform test (using iodine, I₂, in the presence of sodium hydroxide, NaOH) is a specific chemical test used to identify the presence of methyl ketones or secondary alcohols that can be oxidized to methyl ketones.
When a methyl ketone like acetone reacts with I₂/NaOH, it undergoes halogenation followed by cleavage to form a yellow precipitate of iodoform (CHI₃) and the sodium salt of the corresponding carboxylic acid:
$C{H}_{3}COC{H}_3+3I_2+4NaOH\to C{H}_{3}COONa+CH{I}_3\downarrow \left(\text{yellow precipitate}\right)+3NaI+3H_{2}O$

Since benzophenone does not possess a methyl group attached to the carbonyl carbon ($-COC{H}_3$), it does not undergo the iodoform reaction and does not form any yellow precipitate under these conditions.

Let us briefly look at why the other options cannot distinguish them:
- Tollen's reagent and Fehling's reagent are mild oxidizing agents used to distinguish aldehydes from ketones. Since both acetone and benzophenone are ketones, neither reacts with these reagents.
- 2,4-DNP (2,4-dinitrophenylhydrazine) reagent reacts with the carbonyl group of both aldehydes and ketones to form colored precipitates (hydrazones). Since both acetone and benzophenone contain carbonyl groups, both will give a positive test, making it impossible to distinguish between them using this reagent.

Therefore, I₂/NaOH is the only reagent that successfully distinguishes acetone (which gives a yellow precipitate) from benzophenone (which does not react).