On warming, 3-bromobut-1-ene and 1-bromobut-2-ene interconvert in a polar medium. The common species through which they interconvert is

Aa free carbene
Ba cyclic bromonium ion with no allylic delocalisation
Cthe resonance-stabilised allylic cation $CH X_{3} - CH - CH - CH X_{2}$ (+ charge delocalised) formed by ionisation of $C - Br$
Da localised carbanion formed by loss of $H X^{+}$

Answer: C

Ionisation of the

C - Br

bond in either adduct gives the same allylic cation, which recombines with

Br X^{-}

at either terminus. This reversibility lets equilibrium (thermodynamic) control operate.