The compound that is NOT a common component of photochemical smog is:

Photochemical smog is produced when primary pollutants emitted from automobiles—mainly hydrocarbons ($$HC$$) and nitrogen oxides ($$NO_x$$)—interact under the influence of sunlight. The important reactions begin with the photolysis of nitrogen dioxide:

We have the initiation step

$$NO_2 \;\xrightarrow{h\nu}\; NO + O$$

The nascent oxygen atom formed in this step is highly reactive. It combines with molecular oxygen already present in the atmosphere to give ozone:

$$O + O_2 \;\longrightarrow\; O_3$$

Ozone, in turn, reacts with unburnt hydrocarbons and with $$NO$$, leading to the formation of still more secondary pollutants. Among the most prominent secondary pollutants are:

• $$O_3$$ (ozone itself, formed as shown above).
• $$\text{PAN}$$, i.e. peroxyacetyl nitrate, whose condensed structural formula is $$H_3C\!-\!C(=O)\!-\!OONO_2$$. It arises from the reaction of acyl peroxy radicals with $$NO_2$$.
• Reactive aldehydes such as acrolein, $$CH_2 = CHCHO$$, produced by the oxidation of certain unsaturated hydrocarbons.

All three of these species—$$O_3$$, $$\text{PAN}$$, and $$CH_2 = CHCHO$$—are therefore common constituents of photochemical smog.

Now we examine the remaining choice, $$CF_2Cl_2$$ (dichlorodifluoromethane, also called Freon-12). This chlorofluorocarbon is widely discussed in the context of stratospheric ozone depletion, but it is not formed in, nor is it characteristic of, the complex mixture called photochemical smog that develops in the lower troposphere over large cities. Freons are released mainly from refrigerants and aerosol propellants and migrate slowly to the stratosphere; they are not typical products of automobile exhaust, do not participate in the above chain of reactions, and hence do not accumulate in urban smog.

Therefore, among the four species listed, $$CF_2Cl_2$$ is the only one that is not a usual component of photochemical smog.

Hence, the correct answer is Option B.