Electrophilic addition to alkynes with HBr

Steps:

• Compound R is an alkyne, undergoing electrophilic addition with HBr.
• First HBr adds following Markovnikov's rule, forming a vinyl halide intermediate.
• Excess HBr leads to a second addition, yielding a geminal dibromide.
• The major product is the 1,1-dibromoalkane from double Markovnikov addition.

Why A is correct:

• A shows the geminal dibromide, consistent with Markovnikov addition to terminal alkynes per HBr's mechanism.

Why the others are wrong:

• B is the monobromoalkene, incorrect as excess HBr drives to dibromide.
• C is the anti-Markovnikov monobromoalkene, wrong without peroxides.
• D is the vicinal dibromide, incorrect as additions to alkynes yield geminal products.

Not enough information on compound R structure.

Final answer: A