Reactivity order for SN1
`(i) CH_(3)-underset(Cl)underset(|)(CH)-CH_(2)-CH_(3)`
(ii) `Ph-CH_(2)-underset(Cl)underset(|)(CH)-CH_(3)`
(iii) `CH_(3)-overset(Cl)overset(|)underset(OCH_(3))underset(|)C-CH_(3)`
(iv) `CH_(3)CH_(2)-CH_(2)-Cl`

Options

A. `I gt ii gt iii gt iv`
B. `ii gt I gt iii gt iv`
C. `iii gt ii gt I gt iv`
D. `iv gt iii gt ii gt i`

Explanation

To determine the reactivity order for the SN1 reaction of the given compounds, we need to analyze the stability of the carbocations formed during the reaction. The stability of carbocations is influenced by factors such as degree of substitution (primary, secondary, tertiary), resonance, and inductive effects.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (i) CH₃-CH(Cl)-CH₂-CH₃
- (ii) Ph-CH₂-CH(Cl)-CH₃
- (iii) CH₃-CH(Cl)-OCH₃
- (iv) CH₃-CH₂-CH₂-Cl

2. **Determine the Type of Carbocation Formed**:
- (i) **Compound 1**: Upon losing Cl, a secondary carbocation (2°) is formed at the carbon where Cl was attached.
- (ii) **Compound 2**: This also forms a secondary carbocation (2°) at the carbon where Cl was attached, but it is adjacent to a phenyl group which can stabilize the carbocation through resonance.
- (iii) **Compound 3**: This forms a secondary carbocation (2°) at the carbon where Cl is attached, and it is stabilized by the +M (mesomeric) effect from the OCH₃ group.
- (iv) **Compound 4**: This forms a primary carbocation (1°) at the carbon where Cl is attached, which is the least stable.

3. **Evaluate Carbocation Stability**:
- **Compound 3**: The carbocation is stabilized by the +M effect from the OCH₃ group, making it the most stable.
- **Compound 2**: The carbocation is stabilized by resonance with the phenyl group, making it more stable than Compound 1.
- **Compound 1**: The secondary carbocation is less stable compared to the ones in Compounds 2 and 3.
- **Compound 4**: The primary carbocation is the least stable.

4. **Order of Reactivity for SN1**:
- Based on the stability of the carbocations formed, the order of reactivity for the SN1 reaction is:
- (iii) > (ii) > (i) > (iv)
- In numerical terms: 3 > 2 > 1 > 4

5. **Final Answer**:
- The correct order of reactivity for the given compounds in SN1 reactions is **3214**.