Introduction:
Vicinal and geminal dihalides are types of organic compounds that contain two halogen atoms (such as chlorine, bromine, or iodine) attached to adjacent or same carbon atoms, respectively. These compounds can be distinguished by various chemical reactions. However, in this case, we need to determine which of the given reactions does not differentiate between vicinal and geminal dihalides.

Explanation:

KCN/hydrolysis:
- Vicinal dihalides, which have halogen atoms attached to adjacent carbon atoms, can undergo nucleophilic substitution reactions with KCN (potassium cyanide) followed by hydrolysis.
- This reaction leads to the formation of a nitrile (RCN) when the halogen atoms are attached to different carbon atoms.
- On the other hand, geminal dihalides, which have halogen atoms attached to the same carbon atom, do not undergo this reaction as there is no adjacent carbon atom to react with KCN.
- Therefore, this reaction can be used to distinguish vicinal and geminal dihalides.

Alcoholic KOH:
- Alcoholic KOH (potassium hydroxide) is used to differentiate between primary, secondary, and tertiary alkyl halides.
- It promotes the elimination reaction known as the E2 mechanism.
- In the case of vicinal dihalides, both halogen atoms are attached to adjacent carbon atoms, and the reaction with alcoholic KOH can lead to the formation of an alkene.
- However, geminal dihalides, which have halogen atoms attached to the same carbon atom, do not undergo this reaction as there is no adjacent carbon atom to eliminate a halide from.
- Therefore, this reaction can also be used to distinguish vicinal and geminal dihalides.

Aq. KOH/Tollens reagent:
- Aq. KOH (aqueous potassium hydroxide) and Tollens reagent (ammoniacal silver nitrate) are used to distinguish between aldehydes and ketones.
- Aldehydes can be oxidized to carboxylic acids by Tollens reagent, whereas ketones do not react.
- Neither vicinal nor geminal dihalides are aldehydes or ketones, so this reaction is not relevant to distinguishing between them.

Conclusion:
In conclusion, the reaction with alcoholic KOH (option B) does not distinguish between vicinal and geminal dihalides. Both vicinal and geminal dihalides do not undergo elimination reactions with alcoholic KOH, as geminal dihalides lack adjacent carbon atoms. The other reactions mentioned (KCN/hydrolysis and aq. KOH/Tollens reagent) can be used to differentiate between vicinal and geminal dihalides based on their reactivity.