The correct order of `S_N 2//E2` ratio for the % yield of product of the following halide is-
(P)`CH_3-CH-undersetunderset(I)(|)oversetoverset(Ph)(|)C-CH_3` , (Q)`CH_3-undersetunderset(Ph)(|)CH-undersetunderset(I)(|)CH-CH_3`
(R)`CH_3-CH_2-I` , (S)`CH_3-CH-undersetundersetI|CH-CH_3`

Options

A. RgtSgtQgtP
B. RgtQgtSgtP
C. PgtRgtSgtQ
D. QgtPgtRgtS

Explanation

To determine the correct order of the `S_N2/E2` ratio for the percentage yield of products from the given halides (P, Q, R, and S), we need to analyze each compound based on the factors that influence the `S_N2` and `E2` mechanisms.

### Step-by-Step Solution:

1. **Identify the Structure of Each Halide:**
- **P:** `CH3-CH(I)(Ph)-C-CH3` (Tertiary halide)
- **Q:** `CH3-(Ph)(I)-CH-CH3` (Secondary halide with a phenyl group)
- **R:** `CH3-CH2-I` (Primary halide)
- **S:** `CH3-CH(I)-CH-CH3` (Secondary halide)

2. **Determine the Degree of Each Halide:**
- **R** is a primary halide, which is most favorable for the `S_N2` mechanism.
- **S** is a secondary halide, which can undergo both `S_N2` and `E2`, but is more favorable for `E2` due to branching.
- **Q** is a secondary halide but has a phenyl group, which decreases the `S_N2` reactivity.
- **P** is a tertiary halide, which is least favorable for `S_N2` and more favorable for `E2`.

3. **Analyze the Factors Affecting `S_N2` and `E2`:**
- **For `S_N2`:** The reaction is favored in primary halides and decreases in secondary and tertiary halides.
- **For `E2`:** The reaction is favored in more substituted (branched) alkyl halides.

4. **Rank the Halides Based on `S_N2/E2` Ratio:**
- **R** (Primary halide) will have the highest `S_N2/E2` ratio.
- **S** (Secondary halide) will have the next highest ratio, but still significant `E2` reactivity.
- **Q** (Secondary halide with phenyl) will have a lower `S_N2` ratio due to steric hindrance.
- **P** (Tertiary halide) will have the lowest `S_N2/E2` ratio as it favors `E2`.

5. **Final Order:**
The correct order of `S_N2/E2` ratio from highest to lowest is:
- R > S > Q > P

### Conclusion:
The order of `S_N2/E2` ratio for the percentage yield of products from the halides is:
**R > S > Q > P**