Which of the following species will be least stable?

Options

A.
B.
C.
D.

Explanation

To determine which of the given species is the least stable, we will analyze each compound based on their aromaticity. Aromatic compounds are more stable, while anti-aromatic compounds are the least stable.

### Step-by-Step Solution:

1. **Identify the Compounds**: We have four compounds to analyze. Let's label them for clarity:
- Compound A
- Compound B
- Compound C
- Compound D

2. **Check for Aromaticity**: To classify each compound, we will check if they are cyclic, planar, and have a conjugated system of π electrons.
- **Aromatic Compounds**: Must have (4n + 2) π electrons.
- **Anti-aromatic Compounds**: Must have 4n π electrons.

3. **Analyze Compound A**:
- **Cyclic**: Yes
- **Planar**: Yes
- **Conjugated**: Yes
- **π Electrons**: 6 (which is 4n + 2, where n=1)
- **Conclusion**: Aromatic and stable.

4. **Analyze Compound B** (Cyclooctatetraene):
- **Cyclic**: Yes
- **Planar**: Yes
- **Conjugated**: Yes
- **π Electrons**: 10 (which is 4n + 2, where n=2)
- **Conclusion**: Aromatic and stable.

5. **Analyze Compound C**:
- **Cyclic**: Yes
- **Planar**: Yes
- **Conjugated**: Yes
- **π Electrons**: 6 (which is 4n + 2, where n=1)
- **Conclusion**: Aromatic and stable.

6. **Analyze Compound D**:
- **Cyclic**: Yes
- **Planar**: Yes
- **Conjugated**: Yes
- **π Electrons**: 4 (which is 4n, where n=1)
- **Conclusion**: Anti-aromatic and least stable.

7. **Final Comparison**:
- Compounds A, B, and C are aromatic and thus more stable.
- Compound D is anti-aromatic and therefore the least stable.

### Answer:
The least stable compound is **Compound D** (anti-aromatic).

---