`+I` effect is shown by:

Options

A. `-NO_(2)`
B. `-Cl`
C. `-Br`
D. `-CH_(3)`

Answer

To determine which group exhibits the +I (positive inductive) effect, we need to analyze the given options based on their electronegativity relative to carbon. The +I effect refers to the electron-donating ability of a substituent through a covalent bond.

### Step-by-Step Solution:

1. **Understanding the +I Effect**:
- The +I effect is the ability of a substituent to donate electron density to the carbon atom it is attached to. This occurs through a covalent bond.
- Groups that are less electronegative than carbon can exhibit the +I effect.

2. **Analyzing the Given Options**:
- **Option 1: NO2 (Nitro group)**:
- The nitrogen in the NO2 group is more electronegative than carbon. Therefore, it withdraws electron density from carbon.
- This means NO2 exhibits a -I effect (negative inductive effect), not +I.

- **Option 2: Cl (Chlorine)**:
- Chlorine is also more electronegative than carbon. Thus, it will withdraw electron density from the carbon atom.
- This means Cl exhibits a -I effect, not +I.

- **Option 3: Br (Bromine)**:
- Bromine, like chlorine, is more electronegative than carbon. Therefore, it will also withdraw electron density.
- This means Br exhibits a -I effect, not +I.

- **Option 4: CH3 (Methyl group)**:
- The methyl group (CH3) has carbon atoms that are less electronegative than the carbon to which it is attached. Therefore, it donates electron density to the carbon atom.
- This means CH3 exhibits a +I effect (positive inductive effect).

3. **Conclusion**:
- Among the given options, only the CH3 group shows the +I effect. Therefore, the correct answer is **Option D: CH3**.