Electrophilic addition via chloronium ion

Steps:

• The pi electrons of 2-methylpropene attack Cl2, forming a three-membered chloronium ion intermediate and releasing Cl-.
• The chloronium ion bridges the double bond, with positive charge more on the tertiary carbon due to stability.
• Cl- attacks the more substituted carbon from the opposite side, leading to anti addition.
• The product is 2-chloro-1-chloro-2-methylpropane, following Markovnikov orientation.

Why B is correct:

• B accurately describes the chloronium ion as a bridged intermediate, per the standard mechanism for alkene halogenation where the electrophile forms a cyclic ion to explain stereospecific anti addition.

Why the others are wrong:

• A incorrectly suggests a free carbocation, ignoring the bridged structure that prevents rearrangement.
• C wrongly claims syn addition, violating the back-side attack in SN2-like step.
• D misstates the rate-determining step as nucleophilic attack, when it's actually electrophile addition.

Final answer: B