Among $\mathrm{F}{}^{\bullet }$, $\mathrm{Cl}{}^{\bullet }$ and $\mathrm{Br}{}^{\bullet }$, the radical that abstracts hydrogen from an alkane with the highest selectivity is

Reactivity and selectivity vary inversely. $\mathrm{F}{}^{\bullet }$ is the most reactive and almost non-selective, $\mathrm{Cl}{}^{\bullet }$ is intermediate, and $\mathrm{Br}{}^{\bullet }$ is the least reactive and therefore the most selective.