Which one of the following compounds possesses the most acidic hydrogen?

Options

A. `H_(3)C-C-=C-H`
B.
C.
D.

Explanation

To determine which compound possesses the most acidic hydrogen, we need to analyze the compounds based on their functional groups and the effects they exert on acidity. The acidity of a hydrogen atom in a compound is influenced by the stability of the conjugate base formed when that hydrogen is removed.

### Step-by-Step Solution:

1. **Identify the Compounds**:
Let's denote the four compounds as A, B, C, and D. Each compound will have a different functional group that influences its acidity.

2. **Understand the Effects**:
The acidity of a hydrogen atom is enhanced by electron-withdrawing groups that stabilize the negative charge of the conjugate base. The two main effects to consider are:
- **Inductive Effect (-I)**: The permanent polarization of a bond due to electronegative atoms.
- **Mesomeric Effect (-M)**: The delocalization of electrons through resonance.

3. **Rank the Functional Groups**:
We need to determine which functional group has the strongest -I or -M effect. The general order of acidity based on functional groups is:
- **Cyanide (CN)**: Strong -M effect due to high electronegativity.
- **Carboxylic Acid (COOH)**: Moderate -M effect, stabilizes the conjugate base.
- **Ketone (C=O)**: Weaker -M effect compared to carboxylic acids.
- **Other groups**: Depending on their structure, they may have weaker effects.

4. **Analyze Each Compound**:
- For compound C (Cyanide), the conjugate base formed after losing the hydrogen is highly stabilized due to the strong -M effect of the CN group.
- For compound B (Carboxylic Acid), the loss of hydrogen leads to a stable carboxylate ion, which is also stabilized by resonance.
- For compound D (Ketone), the conjugate base is less stable compared to the previous two.

5. **Conclusion**:
Based on the analysis, compound C (Cyanide) possesses the most acidic hydrogen because the resulting conjugate base is the most stable due to the strong -M effect of the CN group.

### Final Answer:
The compound that possesses the most acidic hydrogen is **C (Cyanide)**.