Which of the following statements is correct?

(a) The vinyl carbocation from 2-butyne is more stable because of the presence of electron-repelling ${\mathrm{CH}}_3$ group attached to ${\mathrm{C}}^{+}\text{(i.e. }{\mathrm{CH}}_3-\stackrel{+}{\mathrm{C}}=\mathrm{CH}-{\mathrm{CH}}_3\text{ ). }$Thus, 2-butyne is soluble in concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ due to the  formation of the salt $\left({\mathrm{CH}}_3-\stackrel{+}{\mathrm{C}}=\mathrm{CH}-{\mathrm{CH}}_3\right)\left({\mathrm{HSO}}_4^{-}\right).$

(b) The addition of HBr proceeds through the formation of a carbocation. The alkyl carbocation formed from the alkene group $\left({\mathrm{H}}_3\mathrm{C}-{\stackrel{+}{\mathrm{C}}\mathrm{HCH}}_2\mathrm{C}\equiv \mathrm{CH}\right)$ is more stable than the vinyl carbocation$\left({\mathrm{CH}}_2={\mathrm{CHCH}}_2\stackrel{+}{\mathrm{C}}={\mathrm{CH}}_2\right)$ from the alkyne group. The $\mathrm{\Delta }{H}^{‡}$ for the former is lesser than that of the latter resulting in the addition to HBr across the double bond, i.e. the alkene reacts at a faster rate than the alkyne towards electrophilic addition. Thus, the reaction taking place is

$$\begin{gathered} {\mathrm{H}}_2\mathrm{C}={\mathrm{CHCH}}_2\mathrm{C}\equiv \mathrm{CH}+\mathrm{HBr}⟶{\mathrm{H}}_3\mathrm{C}\underset{\overline{) }}{\mathrm{C}}{\mathrm{HCH}}_2\mathrm{C}\equiv \mathrm{CH} \\ \mathrm{Br} \end{gathered}$$

(c) The addition of ${\mathrm{Br}}_2$ across a double or triple bond occurs via the formation of the three membered ring of bromonium ion.

The brominium ion from alkyne is more strained due to the presence of double bond (shorter bond length). Moreover, its carbon atoms have more s-character. These factors make this brominium ion less stable than that from the alkene makaing alkynes less reactive than alkenes.

( d) The hydrogenation proceeds via the adsorption on the surface of the catalyst. The adsorption of alkene occurs when the plane of the $\mathrm{\pi }$-bond approaches perpendicular to the catalyst. The alkyne, any direction (parallel to C-C bond) can lead to the adsorption due to the cylindrical nature of the $\mathrm{\pi }$-bonds. Thus, the transition state in case of alkynes is less constrained leading to the more positive value of $\mathrm{\Delta }{S}^{‡}$. This makes alkynes to react at a faster rate than alkenes.