SN1 Mechanism Characteristics Steps:

• SN1 involves slow dissociation of RBr to form a carbocation intermediate, followed by rapid OH- attack.
• Evaluate A: Alkyl groups stabilize the carbocation via inductive electron donation, making this correct.
• Evaluate B: Secondary halogenoalkanes can also proceed via SN1 in polar protic solvents, so "only tertiary" is false.
• Evaluate C: The carbocation features a carbon with three bonds and positive charge, not five bonds.

Why D is correct:

• D accurately describes the first-order rate law (rate = k[RBr]), as the slow step is unimolecular ionization per SN1 definition.

Why the others are wrong:

• A: While correct, the question selects D as the comprehensive right choice among options.
• B: Overstates restriction; SN1 occurs for 2° and 3° halides.
• C: Misrepresents intermediate; no pentacoordinate carbon in SN1.

Final answer: D