For the peroxide-initiated addition of HBr to propene, which of the following steps are part of the mechanism?

Initiation is homolysis of the peroxide. In propagation $\mathrm{Br}\cdot$ adds to the terminal carbon to give the secondary radical, which then abstracts H from HBr to regenerate $\mathrm{Br}\cdot$. Addition at the middle carbon would give a less stable primary radical, so step D does not occur.