Chain transfer reagent is:

Options

A. `C Cl_(4)`
B. `CH_(4)`
C. `O_(2)`
D. `H_(2)`

Explanation

To solve the question "Chain transfer reagent is:", we need to understand what a chain transfer reagent is and identify which of the given options fits this definition.

### Step-by-Step Solution:

1. **Definition of Chain Transfer Reagent**:
A chain transfer reagent is a substance that can transfer a radical to a growing polymer chain during chain polymerization. This process results in the formation of a new radical, which can initiate the growth of a new polymer chain.

2. **Understanding the Options**:
The options provided are:
- CCl4 (Carbon Tetrachloride)
- CH4 (Methane)
- O2 (Oxygen)
- H2 (Hydrogen)

3. **Analyzing Each Option**:
- **CCl4**: This compound can break down to form a chlorine radical (Cl•) and a trichloromethyl radical (CCl3•). The presence of these radicals allows CCl4 to act as a chain transfer reagent by transferring the radical to the growing polymer chain.
- **CH4**: Methane does not produce radicals that can initiate or transfer chains in polymerization processes.
- **O2**: Oxygen primarily acts as an oxidizing agent and does not function as a chain transfer reagent in the context of polymerization.
- **H2**: Hydrogen gas does not participate in chain transfer reactions in polymerization.

4. **Conclusion**:
Based on the analysis, the only option that fits the definition of a chain transfer reagent is **CCl4**. Therefore, the correct answer is **CCl4**.

### Final Answer:
The chain transfer reagent is **CCl4**.

---