Reaction of carbonyl compound with one of the following reagents involves nucleophilic addition followed by elimination of water. The reagent is:

Options

A. hydrocyanic acid
B. sodium hydrogen sulphite
C. a grignard reagent
D. hydrazine in presence of feebly acidic solution

Explanation

To solve the question regarding which reagent reacts with a carbonyl compound involving nucleophilic addition followed by the elimination of water, we can analyze the options provided:

### Step-by-Step Solution:

1. **Identify the Nature of the Reaction**:
- The question specifies that the reaction involves nucleophilic addition followed by the elimination of water. This indicates that we are looking for a reagent that can act as a nucleophile and form a stable intermediate that can subsequently lose a water molecule.

2. **Evaluate the Reagents**:
- **Option 1: Hydrocyanic Acid (HCN)**:
- HCN can act as a nucleophile due to the presence of the cyanide ion (CN⁻). However, the reaction with carbonyl compounds typically leads to the formation of cyanohydrins and does not involve the elimination of water.
- **Option 2: Sodium Hydrogen Sulfide (NaHS)**:
- NaHS can provide HS⁻ as a nucleophile, but it does not lead to a reaction that involves the elimination of water. Instead, it forms thioethers or thiols.
- **Option 3: Grignard Reagent (RMgX)**:
- Grignard reagents are strong nucleophiles and react with carbonyl compounds to form alcohols. However, this reaction does not involve the elimination of water.
- **Option 4: Hydrazine (NH2NH2)**:
- Hydrazine acts as a nucleophile and can react with carbonyl compounds to form hydrazones. The reaction involves the nucleophilic addition of the NH2 group to the carbonyl carbon, forming an intermediate. This intermediate can then lose a water molecule, resulting in the formation of a double bond between carbon and nitrogen.

3. **Conclusion**:
- Based on the analysis, the reagent that fits the criteria of nucleophilic addition followed by the elimination of water is **Option 4: Hydrazine (NH2NH2)**.

### Final Answer:
The correct reagent is **Hydrazine (NH2NH2)**.