The higher concentration of which gas in air can cause stiffness of flower buds?

We begin by recalling basic plant physiology. Flower buds are delicate tissues that require an uninterrupted supply of water and nutrients through the xylem and phloem. Any air pollutant that interferes with cellular respiration, enzyme activity, or membrane permeability can harden or stiffen these buds.

Among the common atmospheric pollutants, we normally consider $$NO,\; CO,\; CO_2,$$ and $$SO_2.$$ Each gas affects plants in its own characteristic way:

We have $$CO_2,$$ which, although it can lead to increased acidity in very high concentrations, is also a raw material for photosynthesis. Moderate elevations of $$CO_2$$ generally enhance, rather than hinder, plant growth. Therefore, excessive stiffness of buds is not typically attributed to $$CO_2.$$

Next, $$CO$$ is a product of incomplete combustion. While it is toxic to animals because it binds with haemoglobin to form carboxyhaemoglobin, its direct impact on plants is comparatively minor. It does not usually cause mechanical stiffening of plant tissues.

Similarly, $$NO$$ (nitric oxide) can participate in photochemical smog formation and may lead to chlorosis (loss of green colour) at high levels, yet it is not known to induce bud stiffness specifically.

The remaining gas is $$SO_2.$$ We recall from plant‐air‐pollution studies that sulfur dioxide diffuses into leaf tissues through stomata. Inside the moist cellular environment, $$SO_2$$ hydrates to form sulfite and bisulfite ions:

$$SO_2 + H_2O \rightarrow HSO_3^- + H^+$$

These ions are toxic; they attack cell membranes, denature proteins, and inhibit enzyme systems. One conspicuous external symptom is loss of turgor followed by a rigid, hardened state in young tissues, especially in flower buds, because cell wall plasticity is reduced. The phenomenon is described in horticultural literature as “bud stiffness” or “hardening.”

Hence, among the four gases listed, a higher concentration of $$SO_2$$ is specifically associated with the stiffness of flower buds.

Therefore, the gas responsible is $$SO_2,$$ corresponding to Option C (option number 3 in the given ordering).

Hence, the correct answer is Option C.