The higher concentration of which gas in air can cause stiffness of flower buds?
(A) $S{{O}_{2}}$
(B) $N{{O}_{2}}$
(C) $C{{O}_{2}}$
(D) CO

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Hint: Acid rain is precipitation that has the pH which is less than that of the pH of natural rainwater. This is the reason behind the stiffness of flower buds. The pH determines whether the solution is acidic or basic.

Complete step by step solution:
The presence of harmful pollutants in the air leads to pollution. Human activities like the burning of fossil fuels, the smoke from the factories, thermal power plants etc. leads to air pollution. The process of combustion produces carbon monoxide, nitrogen oxide. These gases contribute to air pollution. When sulphur dioxide and the nitrogen oxides present in the atmosphere mix with water vapours, it forms Sulphuric acid or nitric acid or it leads to acid rain. So, due to the acid rain in plants, a high concentration of sulphur dioxide i.e. $S{{O}_{2}}$ makes the flower buds stiff and it makes them fall off which hampers the reproductive cycle of the plant.

Hence the correct answer is (A) i.e. $S{{O}_{2}}$

Additional information:
Acid rain have several harmful effects such as:
-It increases the pH of the water bodies which make it difficult for the survival of the aquatic animals.
-It affects the food chain.
-It also causes harm to the forests by damaging the leaves of the trees and by removing the nutrients from the soil. It makes it hard for the trees to take up water through the steam.

Note: By designing a cleaner power plant we can prevent air pollution. And if we reduce the use of fossil fuel, we can reduce the emission of harmful pollutants which leads to acid rain.