The most unstable carbocation is:

Options

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

Explanation

To determine the most unstable carbocation among the given options, we need to analyze the stability of each carbocation based on the substituents attached to the carbon atom bearing the positive charge.

### Step-by-Step Solution:

1. **Identify the Carbocations**:
- A) CH3-CH2^+
- B) CH2Cl-CH2^+
- C) CH2^+-CHO
- D) CH2^+-O-CH3

2. **Analyze Stability Factors**:
- Carbocations are stabilized by electron-donating groups (which can donate electron density) and destabilized by electron-withdrawing groups (which pull electron density away).

3. **Evaluate Each Carbocation**:
- **Option A (CH3-CH2^+)**: The ethyl group (CH3-CH2) is an electron-donating group (due to the +I effect), which provides some stability to the carbocation.

- **Option B (CH2Cl-CH2^+)**: The chlorine atom has a -I effect (electron-withdrawing) but is somewhat distant from the carbocation. The CH2 group can provide some stabilization, but overall, it is less stable than option A.

- **Option C (CH2^+-CHO)**: The positive charge is on a carbon adjacent to a carbonyl group (C=O). The oxygen atom is highly electronegative and exerts a strong -I effect, pulling electron density away from the carbocation, making it highly unstable.

- **Option D (CH2^+-O-CH3)**: Here, the positive charge is on a carbon adjacent to an oxygen atom with a lone pair. The oxygen can donate electron density (through resonance), which provides some stabilization to the carbocation.

4. **Conclusion**:
- Among the four options, the carbocation in **Option C (CH2^+-CHO)** is the most unstable due to the strong electron-withdrawing effect of the carbonyl group (C=O), which destabilizes the positive charge.

### Final Answer:
The most unstable carbocation is **Option C (CH2^+-CHO)**.