Rearrangement reactions are shown by :

Options

A. Carbanion
B. Free radical
C. Carbene
D. Carbocation

Explanation

To solve the question regarding which species show rearrangement reactions, we can analyze the options provided: carbanion, free radical, carbene, and carbocation.

### Step-by-Step Solution:

1. **Understanding Rearrangement Reactions**:
- Rearrangement reactions involve the structural reorganization of a molecule, typically to form a more stable intermediate. This is often seen in reaction mechanisms where the stability of the intermediate plays a crucial role.

2. **Identify the Species**:
- The species mentioned are:
- **Carbanion**: A negatively charged carbon species.
- **Free Radical**: A species with an unpaired electron.
- **Carbene**: A neutral species with a divalent carbon atom.
- **Carbocation**: A positively charged carbon species.

3. **Analyzing Each Species**:
- **Carbanion**: Generally, carbanions do not undergo rearrangement reactions because they are already in a relatively stable configuration and tend to react by nucleophilic attack rather than rearrangement.
- **Free Radical**: Free radicals can participate in reactions but do not typically undergo rearrangements as seen with carbocations.
- **Carbene**: Carbenes are neutral and can undergo some rearrangements, but they are not as common as carbocation rearrangements.
- **Carbocation**: Carbocations are known for undergoing rearrangement reactions. They can shift from an unstable state to a more stable state through various mechanisms such as methyl shifts, hydride shifts, or ion shifts.

4. **Conclusion**:
- Among the options provided, **carbocations** are the primary species that undergo rearrangement reactions. Therefore, the correct answer is **carbocation**.

### Final Answer:
The correct option is **carbocation**.