$Br^{\bullet}$ will abstract which of the hydrogen most readily?

Which of the following does not react with chlorine in the dark?

Choose the chain terminating step.
$(1)$ $H_2 \to H^{\bullet} + H^{\bullet}$
$(2)$ $Br_2 \to Br^{\bullet} + Br^{\bullet}$
$(3)$ $Br^{\bullet} + HBr \to H^{\bullet} + Br_2$
$(4)$ $H^{\bullet} + Br_2 \to HBr + Br^{\bullet}$
$(5)$ $Br^{\bullet} + Br^{\bullet} \to Br_2$

Which of the following is a correct reaction?

Compound $A$ on reaction with chlorine in the presence of $U.V.$ light gives $B$; which when reacted with $NaNO_{2}$ in the solvent $N, N$-dimethylformamide $(DMF)$,gives $2$-nitrobutane. The compound $A$ is:

Different hydrogen atoms in the compound are represented by alphabets:
$CH_3(A)-CH=CH(B)-CH_2(C)-CH_2(D)-CH(E)(CH_3)_2(F)$
Arrange them in decreasing order of reactivity towards free radical substitution.