Alkyl halides reacts with $$Na$$  in presence of dry ether to form alkanes. This reaction is known as
Wurtz reaction

In the given question $$t$$ - butyl chloride $${C_4}{H_9}Cl$$   is $$A.$$  It reacts with $$Na$$ metal in dry ether to form a hydrocarbon that on chlorination gives only one monochloro derivative.

Freons or chlorofluoro carbons are responsible for depletion of the ozone layer in the upper strata of the atmosphere. They are used as propellants, aerosol spray caps, refrigerants, fire fighting reagents, etc. They are stable and chemically inert compounds. They absorb $$UV$$ - radiation and break down liberating free atomic chlorine which causes decomposition of ozone through free radical reaction. This results in the depletion of the ozone layer.
They form free radical of chlorine in presence of $$UV$$ - radiation. Such free radical decomposes $${O_3}$$  as follows :
\[\begin{align} & C{{l}^{\bullet }}+{{O}_{3}}\to Cl{{O}^{\bullet }}+{{O}_{2}} \\ & Cl{{O}^{\bullet }}+{{O}_{3}}\to \underset{\begin{smallmatrix} \text{Chlorine free} \\ \,\,\,\,\,\,\,\text{radical} \end{smallmatrix}}{\mathop{C{{l}^{\bullet }}}}\,+2{{O}_{2}} \\ \end{align}\]

Steric congestion around the carbon atom undergoing the inversion process will slow down the $${S_N}2$$  reaction, hence less congestion faster will the reaction. So, the order is
$$C{H_3}Cl > \left( {C{H_3}} \right)C{H_2} - Cl > {\left( {C{H_3}} \right)_2}CH - Cl > {\left( {C{H_3}} \right)_3}CCl$$

TIPS/Formulae :
The reaction proceeds via free radical mechanism.
As $${2^ \circ }$$ free radical is more stable than $${1^ \circ },$$  so $$C{H_3}C{H_2}CH\left( {Br} \right)C{H_3}$$     would be formed.