Addition of HBr to propene gives 2 - bromo propane, this reaction is intiated by

 When no peroxide is added, the first step in the addition of HBr to propene involves addtion of ${\mathrm{H}}^{+}$ ion to form carbocation intermediate. The second step is the addition of bromide ion.

 $\begin{array}{l}{\mathrm{CH}}_2={\mathrm{CHCH}}_3+{\mathrm{H}}^{+}\to {\mathrm{CH}}_3{\mathrm{CH}}^{+}{\mathrm{CH}}_3\\ {\mathrm{CH}}_3{\mathrm{CH}}^{+}{\mathrm{CH}}_3+{\mathrm{Br}}^{-}\to {\mathrm{CH}}_3{\mathrm{CHBrCH}}_3\end{array}$ 

Hence, the correct option (A). ${\mathrm{H}}^{\oplus }$.