The deposition of X and Y on ground surfaces is referred as wet and dry depositions, respectively. X and Y are:

First, let us recall the meanings of the two key terms that appear in the question.

Wet deposition is the process in which substances present in the atmosphere are washed out of the air and brought down to the Earth’s surface by precipitation such as rain, snow, fog or cloud water. Because the material is literally dissolved or suspended in atmospheric water before it reaches the ground, it is called “wet”. Typical examples include acids, nitrates and ammonium compounds that become incorporated into raindrops.

Dry deposition, on the other hand, involves the direct transfer of gases or solid particles from the air to surfaces (soil, vegetation, buildings) without the aid of precipitation. The material settles or is absorbed simply by contact with those surfaces under normal atmospheric motions. Common gases that undergo dry deposition include $$\mathrm{SO_2}$$, $$\mathrm{NO_2}$$ and ozone.

With these definitions in mind, we examine the species listed in the options.

1. Option A proposes $$X = \mathrm{CO_2}$$ and $$Y = \mathrm{SO_2}$$. Carbon dioxide is not typically removed from the atmosphere by rainfall; it stays dissolved only slightly in raindrops and is mainly exchanged biologically (photosynthesis). Hence $$\mathrm{CO_2}$$ is not a characteristic wet-deposition species. This option therefore conflicts with the definition.

2. Option B proposes $$X =$$ ammonium salts and $$Y = \mathrm{SO_2}$$. Ammonium ions $$\left( \mathrm{NH_4^+} \right)$$ usually arise from atmospheric ammonia that readily dissolves in cloud droplets forming ammonium sulfate, ammonium nitrate, etc. These dissolved salts are efficiently delivered to the surface by rain, fitting perfectly with wet deposition. Sulphur dioxide is a gas that adsorbs onto surfaces or is absorbed by leaves directly from air in the absence of rain, matching dry deposition. Hence this pairing is fully consistent with the two definitions.

3. Option C proposes $$X =$$ ammonium salts and $$Y = \mathrm{CO_2}$$. While the first part (ammonium salts) still agrees with wet deposition, the second part fails because, as argued earlier, $$\mathrm{CO_2}$$ is not generally classified under dry deposition in atmospheric chemistry contexts.

4. Option D proposes $$X = \mathrm{SO_2}$$ and $$Y =$$ ammonium salts. This directly contradicts the chemical behaviour: $$\mathrm{SO_2}$$, being a gas, predominantly reaches surfaces through dry deposition, while ammonium salts are taken down by rainwater. Hence this option reverses the correct roles.

After evaluating all four possibilities, we find that only Option B correctly assigns a predominantly rain-borne species (ammonium salts) to wet deposition and a gaseous species (SO$$_2$$) to dry deposition.

Hence, the correct answer is Option 2.