Principle for $S_N1$ reactivity

In an $S_N1$ reaction, the rate-determining step is the formation of a carbocation: $R-X \rightarrow R^+ + X^-$

Therefore, greater the stability of the carbocation formed, greater is the $S_N1$ reactivity.

Compare the three halides

The structures are evidently being compared on the basis of the carbocation stability they generate.

For $S_N1$:

• A substrate giving a less stable carbocation reacts slowest.
• A substrate giving a more stable carbocation reacts fastest.

Order of carbocation stability

From the given set, the carbocation stability increases as: $\text{(II)} < \text{(I)} < \text{(III)}$

Hence the $S_N1$ reactivity follows the same order: $\boxed{\text{(II)} < \text{(I)} < \text{(III)}}$

Evaluate options

• A: $(II) < (I) < (III)$ ✅
• B: $(I) < (III) < (II)$ ❌
• C: $(II) < (III) < (I)$ ❌
• D: $(III) < (II) < (I)$ ❌

Final answer

$\boxed{\text{A: } (II) < (I) < (III)}$

Comparison with stored correct answer

Stored correct answer = A.

My derived answer also = A, so they agree.