Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R)
Assertion (A) : Photochemical smog causes cracking of rubber.
Reason (R): Presence of ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate in photochemical smog makes it oxidizing.
Choose the most appropriate answer from the options given below:

First, let us recall what photochemical smog is. It is the brown hazy mixture that is produced when primary pollutants such as $$\text{NO}$$ and $$\text{NO}_2$$, emitted from vehicular exhaust, react in the presence of strong sunlight with hydrocarbons present in air. The major secondary products that are finally present in this smog are ozone $$\left(\text{O}_3\right)$$, nitric oxide $$\left(\text{NO}\right)$$, acrolein $$\left(\text{CH}_2\!\!=\!\text{CH}-\text{CHO}\right)$$, formaldehyde $$\left(\text{HCHO}\right)$$ and peroxyacetyl nitrate $$\left(\text{PAN},\ \text{CH}_3\text{COOONO}_2\right)$$.

Now, all these substances—especially ozone and PAN—are very powerful oxidising agents. An oxidising agent has the tendency to remove electrons from, that is, oxidise, the substances it comes in contact with. Natural as well as synthetic rubber contains unsaturated $$\text{C=C}$$ bonds in its long-chain molecules. Oxidising agents attack these double bonds, break the chains and weaken the material. This chemical attack is popularly called “cracking of rubber”.

So we have:

$$\text{O}_3 + \text{Rubber} \;\longrightarrow\; \text{Cracked\ rubber\ fragments}$$

Because photochemical smog supplies the oxidants—$$\text{O}_3,\ \text{PAN},\ \text{NO}$$ etc.—it definitely causes cracking of rubber that is exposed to the atmosphere. Thus the statement in the Assertion (A) is correct.

Reason (R) says that the presence of ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate in photochemical smog makes it oxidising. We have just seen that each of these species is indeed an oxidant (ozone and PAN are very strong; acrolein and formaldehyde are milder but still oxidising). Therefore the Reason (R) is also correct.

Furthermore, we notice that the reason directly supplies the chemical cause behind the cracking mentioned in the assertion: because the smog is oxidising, it attacks the $$\text{C=C}$$ bonds of rubber and cracks it. Therefore (R) is the true and sufficient explanation of (A).

Hence, the correct answer is Option 3.