A bromine atom abstracts one hydrogen atom from the compound shown. Which radical shown is formed most readily?

The compound shown is 3-methylbut-1-ene. Removal of the C3-H gives a radical that is both tertiary and allylic, delocalised over C1 and C3 by resonance with the C=C bond. The primary radical is neither tertiary nor allylic, and the vinylic radicals are sp${}^2$ radicals with no delocalisation, so all are less stable.