`(I) Mel+PH_(3)overset(60% "acetone")overset(+40%H_(2)O)toMeoverset(oplus)(P)H_(3)+I^(Theta)`
`(II) Mel+PH_(3)overset(H_(2)O)toMeoverset(oplus)(P)H_(3)+I^(Theta)`
Increase in solvent polarity stablilises the `T.S`., lowers `E_(act)`, and increases the rate.

Options

A. If both `(A)` and `(R )` are true, but `( R)` is the correct explanation of `(A)`
B. If both `(A)` and `( R)` are true, but `(R )` is not the correct explanation of `(A)`.
C. If `(A)` is true but `( R)` is false.
D. If `(A)` is false but `( R)` is true.

Answer

To solve the problem, we need to analyze the two reactions provided and understand the role of solvent polarity in influencing the reaction rate.

### Step-by-Step Solution:

1. **Identify the Reactions:**
- Reaction (I): Mel + PH₃ in 60% acetone and 40% water produces Me + PH₃ + I⁻.
- Reaction (II): Mel + PH₃ in 100% water produces Me + PH₃ + I⁻.

2. **Determine the Reaction Mechanism:**
- Both reactions are likely following an SN2 mechanism, where a nucleophile (PH₃) attacks the electrophile (Mel), leading to the formation of the product (Me) and the leaving group (I⁻).

3. **Transition State Formation:**
- In an SN2 reaction, a transition state is formed where the nucleophile is partially bonded to the electrophile. The stability of this transition state is crucial for the reaction rate.

4. **Effect of Solvent Polarity:**
- The assertion states that increasing solvent polarity stabilizes the transition state. In polar solvents, the transition state can be better solvated, which lowers the activation energy (Eₐ) required for the reaction to proceed.

5. **Comparison of Reaction Rates:**
- Since reaction (II) occurs in 100% water (a highly polar solvent), it is expected to have a lower activation energy compared to reaction (I) in a less polar solvent (60% acetone and 40% water). Therefore, the rate of reaction (II) is greater than that of reaction (I).

6. **Conclusion:**
- The assertion is correct: the rate of reaction (II) is greater than that of reaction (I) due to the higher polarity of the solvent stabilizing the transition state, thus lowering the activation energy and increasing the rate of reaction.

### Final Answer:
- Both the assertion and reason are correct, and the reason provided is the correct explanation for the assertion.