The reagent which does not react with both, acetone and benzaldehyde

Fehling's solution 

The correct answer is option 3. Fehling's solution.

To determine which reagent does not react with both acetone and benzaldehyde, let's analyze the reactivity of each reagent with these two compounds.

1. Sodium hydrogensulphite \((NaHSO_3)\):

Sodium hydrogensulphite reacts with both ketones and aldehydes to form bisulfite addition compounds. Therefore, it will react with both acetone (a ketone) and benzaldehyde (an aldehyde).

2. Phenyl hydrazine:

Phenyl hydrazine reacts with both ketones and aldehydes to form hydrazones. Therefore, it will react with both acetone and benzaldehyde.

3. Fehling's solution:

Fehling's solution is a reagent used to distinguish between aldehydes and ketones. It reacts with aldehydes to give a positive test (formation of a red precipitate of \(Cu_2O\)), but it does not react with ketones. Therefore, it will react with benzaldehyde but not with acetone.

4. Grignard reagent:

Grignard reagents (RMgX) react with both ketones and aldehydes to form alcohols after hydrolysis. Therefore, it will react with both acetone and benzaldehyde.

The reagent that does not react with both acetone and benzaldehyde is Fehling's solution, as it reacts with benzaldehyde but not with acetone.

Therefore, the correct answer is Fehling's solution.