In which of the following carbocations, delocalisation of positive charge is possible :

Options

A.
B.
C.
D. `overset(o+)(C )H_(2)-O-CH_(3)`

Explanation

To determine in which of the given carbocations delocalization of the positive charge is possible, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Carbocations**:
- A carbocation is a positively charged carbon atom. The stability and reactivity of carbocations can be influenced by the ability to delocalize the positive charge through resonance.

2. **Analyzing Option A**:
- The first option presents a carbocation located at a bridgehead position in a bicyclic structure.
- According to Bredt's rule, a carbocation at a bridgehead position cannot delocalize its positive charge due to steric hindrance and the geometric constraints of the bicyclic system.
- **Conclusion for A**: No delocalization is possible.

3. **Analyzing Option B**:
- The second option shows a carbocation on a benzene ring.
- Although benzene is a stable aromatic compound, the carbocation is located on a sp² carbon, which does not allow for delocalization of the positive charge as it would disrupt the aromaticity.
- **Conclusion for B**: No delocalization is possible.

4. **Analyzing Option C**:
- The third option presents a carbocation that appears to have alternating single and double bonds.
- However, the arrangement does not provide the necessary structure for resonance stabilization (i.e., it lacks the required alternating single and double bonds around the positive charge).
- **Conclusion for C**: No delocalization is possible.

5. **Analyzing Option D**:
- The fourth option shows a carbocation (CH₂⁺) adjacent to an oxygen atom with a lone pair.
- The lone pair on the oxygen can participate in resonance by donating electrons to the positively charged carbon, allowing for delocalization of the positive charge.
- The resulting structure would involve a double bond between the carbon and oxygen, stabilizing the positive charge.
- **Conclusion for D**: Delocalization is possible.

### Final Answer:
The correct option where delocalization of the positive charge is possible is **D**.