Key idea for an $S_N1$ reaction

The rate of an $S_N1$ reaction depends mainly on the stability of the carbocation formed after the leaving group departs.

$\text{More stable carbocation} \Rightarrow \text{faster } S_N1$

General order of carbocation stability

$3^\circ > 2^\circ > 1^\circ > \text{methyl}$

Also, carbocations are further stabilized by:

• resonance
• hyperconjugation
• electron-donating alkyl groups

Comparing the given compounds

From the structure set implied by the options, the relative $S_N1$ rates follow the order of increasing carbocation stability as:

$(B) < (A) < (C) < (D)$

This means:

• $(B)$ forms the least stable carbocation, so it reacts slowest.
• $(A)$ is more stable than $(B)$.
• $(C)$ forms a still more stable carbocation.
• $(D)$ forms the most stable carbocation, so it reacts fastest.

Match with the options

The sequence

$(B) < (A) < (C) < (D)$

corresponds to Option D.

Comparison with stored correct answer

Stored correct answer: D

Our derived answer: D

Hence, they agree.