Which one of the following is least reactive towards HBr?

When hydrocarbons react with HBr (in the absence of peroxide), they follow electrophilic addition reaction where the major product is formed from the highly stable carbocation (acts as an intermediate in the reaction). Alkynes are usually less reactive than alkenes in electrophilic addition because pi-electrons are strongly hold by triple bond than double bond. Also, C atom of $\mathrm{C}\equiv \mathrm{C}$ has more s-character (more electronegative) than C atom involve in double bond.