Halogenation of alkanes is an example of which mechanism?

What is the use of $tetraethyl$ lead $(TEL)$?

Identify the reagent $(A)$ used in the following reaction.

What is the bond length of the $C-C$ bond in an alkane where all carbon atoms are $sp^{3}$ hybridized (in $pm$)?

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 true for the given reaction: $R - H + X_2 \xrightarrow{hv} R - X + H - X$?