A new carbon-carbon bond formation is possible in the following `"reaction" // "reactions"`:

Options

A. `C_(6)H_(6)+CH_(3)Cl overset(AlCl_(3))to`
B. `CH_(3)CH_(2)Br+CH_(3)CH_(2)NH_(2) to`
C. `CH_(3)CH_(2)Br+Na^(+)""^(-)OCH_(3) to`
D. `CH_(3)CH_(2)Br+KCN(alc.) to`

Explanation

To determine whether a new carbon-carbon bond formation is possible in the given reactions, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Option D Analysis**:
- The reaction involves CH3-CH2-Br and KCN (alcoholic).
- In this reaction, the cyanide ion (CN-) acts as a nucleophile.
- The bromine atom (Br) is a good leaving group and will leave, allowing the cyanide to attack the carbon atom bonded to Br.
- This results in the formation of a new carbon-carbon bond between the CH3 and the carbon from CN, leading to the product CH3-CH2-C≡N.
- **Conclusion**: A new carbon-carbon bond is formed in this reaction.

2. **Option C Analysis**:
- The reaction involves CH3-CH2-Br and O-CH3 (methoxy group).
- Here, the methoxy group acts as a nucleophile.
- The methoxy will attack the carbon bonded to Br, causing Br to leave.
- The product formed is CH3-CH2-O-CH3, which results in the formation of a carbon-oxygen bond, not a carbon-carbon bond.
- **Conclusion**: No new carbon-carbon bond is formed.

3. **Option B Analysis**:
- The reaction involves CH3-CH2-Br and NH2-CH3 (methylamine).
- The nitrogen atom in methylamine has a lone pair and acts as a nucleophile.
- It attacks the carbon bonded to Br, leading to the formation of CH3-CH2-NH-CH3 and HBr as a byproduct.
- This results in the formation of a carbon-nitrogen bond, not a carbon-carbon bond.
- **Conclusion**: No new carbon-carbon bond is formed.

4. **Option A Analysis**:
- The reaction involves benzene (C6H6) and CH3Cl in the presence of AlCl3.
- This is a Friedel-Crafts alkylation reaction.
- The CH3 group from CH3Cl will attach to the benzene ring, forming toluene (C6H5-CH3).
- This reaction results in the formation of a carbon-carbon bond between the benzene ring and the CH3 group.
- **Conclusion**: A new carbon-carbon bond is formed.

### Final Conclusion:
The reactions that lead to the formation of new carbon-carbon bonds are:
- **Option D**: CH3-CH2-Br + KCN → CH3-CH2-C≡N (C-C bond formed)
- **Option A**: C6H6 + CH3Cl → C6H5-CH3 (C-C bond formed)

### Summary:
- **Option D** and **Option A** result in the formation of new carbon-carbon bonds.
- **Option B** and **Option C** do not form carbon-carbon bonds.