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(B) The reaction is an electrophilic addition of $HBr$ to propyne $(H_3C-C\equiv CH)$.Step $1$: The first molecule of $HBr$ adds to the triple bond fo

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$H_3C-C(Br)_2-CH_3$

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(B) The reaction is an electrophilic addition of $HBr$ to propyne $(H_3C-C\equiv CH)$.Step $1$: The first molecule of $HBr$ adds to the triple bond following Markownikoff's rule,where the electrophile $H^+$ attaches to the terminal carbon to form a more stable secondary carbocation $(H_3C-C^+=CH_2)$. The bromide ion $(Br^-)$ then attacks the carbocation to form $2-$bromopropene $(H_3C-C(Br)=CH_2)$.Step $2$: The second molecule of $HBr$ adds to the double bond of $2-$bromopropene,again following Markownikoff's rule. The $H^+$ adds to the terminal carbon $(CH_2)$ to form a more stable carbocation $(H_3C-C^+(Br)-CH_3)$,which is stabilized by the resonance effect of the bromine atom. The $Br^-$ then attacks this carbocation to yield $2,2-$dibromopropane $(H_3C-C(Br)_2-CH_3)$ as the major product.

The major product in the following reaction is $H_3C-C\equiv CH + HBr$ (excess)

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