Vicinal dibromide cyclizes with ammonia to form 2-substituted aziridine Steps:

• Analyze coniine's structure as a 2-alkylaziridine formed via double SN2 by NH3 on vicinal dibromide.
• The dibromide's CH2Br becomes one aziridine CH2; the CHBr-R becomes the substituted CH-R.
• Match R to coniine's alkyl substituent: ethyl requires -CH2CH3 on the second carbon.
• Select the dibromide with four-carbon chain ending in ethyl: BrCH2-CHBr-CH2CH3.

Why D is correct:

• BrCH2CHBrCH2CH3 cyclizes to 2-ethylaziridine, matching coniine's formula and structure via ring closure.

Why the others are wrong:

• A: Cyclopentane ring creates bicyclic product, not simple aziridine.
• B: Propyl and cyclopentane yield complex fused ring, mismatched skeleton.
• C: Forms 2-methylaziridine (from -CH3), wrong alkyl chain length.

Final answer: D