Match List - I with List - II:
List - I (compound)                       List - II (effect/affected species)
a. Carbon monoxide                   i.    Carcinogenic
b. Sulphur dioxide                       ii.   Metabolized by pyrus plants
c. Polychlorinated biphenyls     iii.  Haemoglobin
d. Oxides of Nitrogen                 iv.  Stiffness of flower buds
Choose the correct answer from the options given below:

We begin with compound (a) carbon monoxide, whose most well-known toxic action is its strong affinity for the blood pigment haemoglobin. The reaction is written as $$\mathrm{CO + Hb \;\longrightarrow\; HbCO}$$ and the product, called carboxy-haemoglobin, blocks the normal transport of oxygen. Thus the only entry in List-II that fits carbon monoxide is “haemoglobin (iii).”

Next, compound (b) sulphur dioxide is an acidic, hygroscopic gas that injures green vegetation; one characteristic injury noted in horticulture is the abnormal hardening or stiffness of developing flower buds. Therefore sulphur dioxide must be matched with “stiffness of flower buds (iv).”

For compound (c) polychlorinated biphenyls (PCBs), textbooks of environmental chemistry state that these chlorinated aromatic compounds are potent human and animal carcinogens. Hence PCBs correspond to “carcinogenic (i).”

Finally, compound (d) the oxides of nitrogen (NO and NO2) can act as a supplementary nitrogen source for certain higher plants; pear trees belonging to the genus Pyrus are specifically reported to absorb and metabolise these oxides. Consequently, oxides of nitrogen are best paired with “metabolized by pyrus plants (ii).”

Collecting all the matches we have found:

$$\begin{aligned} (a) &\;\;\text{Carbon monoxide} \;\longrightarrow\; (iii)\;\text{Haemoglobin} \\ (b) &\;\;\text{Sulphur dioxide} \;\longrightarrow\; (iv)\;\text{Stiffness of flower buds} \\ (c) &\;\;\text{Polychlorinated biphenyls} \;\longrightarrow\; (i)\;\text{Carcinogenic} \\ (d) &\;\;\text{Oxides of nitrogen} \;\longrightarrow\; (ii)\;\text{Metabolized by pyrus plants} \end{aligned}$$

This sequence corresponds exactly to Option A.

Hence, the correct answer is Option A.